doc-src/Nitpick/nitpick.tex
author blanchet
Tue Oct 26 11:00:17 2010 +0200 (2010-10-26)
changeset 40147 d170c322157a
parent 39359 6f49c7fbb1b1
child 40341 03156257040f
permissions -rw-r--r--
improved English
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\documentclass[a4paper,12pt]{article}
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\usepackage[T1]{fontenc}
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\usepackage{amsmath}
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\usepackage{amssymb}
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\usepackage[english,french]{babel}
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\usepackage{color}
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\usepackage{footmisc}
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\usepackage{graphicx}
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%\usepackage{mathpazo}
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\usepackage{multicol}
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\usepackage{stmaryrd}
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%\usepackage[scaled=.85]{beramono}
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\usepackage{../iman,../pdfsetup}
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%\oddsidemargin=4.6mm
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%\evensidemargin=4.6mm
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%\textwidth=150mm
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%\topmargin=4.6mm
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%\headheight=0mm
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%\headsep=0mm
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%\textheight=234mm
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\def\Colon{\mathord{:\mkern-1.5mu:}}
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%\def\lbrakk{\mathopen{\lbrack\mkern-3.25mu\lbrack}}
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%\def\rbrakk{\mathclose{\rbrack\mkern-3.255mu\rbrack}}
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\def\lparr{\mathopen{(\mkern-4mu\mid}}
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\def\rparr{\mathclose{\mid\mkern-4mu)}}
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\def\unk{{?}}
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\def\undef{(\lambda x.\; \unk)}
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%\def\unr{\textit{others}}
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\def\unr{\ldots}
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\def\Abs#1{\hbox{\rm{\flqq}}{\,#1\,}\hbox{\rm{\frqq}}}
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\def\Q{{\smash{\lower.2ex\hbox{$\scriptstyle?$}}}}
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\hyphenation{Mini-Sat size-change First-Steps grand-parent nit-pick
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counter-example counter-examples data-type data-types co-data-type 
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co-data-types in-duc-tive co-in-duc-tive}
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\urlstyle{tt}
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\begin{document}
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\selectlanguage{english}
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\title{\includegraphics[scale=0.5]{isabelle_nitpick} \\[4ex]
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Picking Nits \\[\smallskipamount]
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\Large A User's Guide to Nitpick for Isabelle/HOL}
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\author{\hbox{} \\
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Jasmin Christian Blanchette \\
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{\normalsize Institut f\"ur Informatik, Technische Universit\"at M\"unchen} \\
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\hbox{}}
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\maketitle
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\tableofcontents
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\setlength{\parskip}{.7em plus .2em minus .1em}
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\setlength{\parindent}{0pt}
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\setlength{\abovedisplayskip}{\parskip}
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\setlength{\abovedisplayshortskip}{.9\parskip}
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\setlength{\belowdisplayskip}{\parskip}
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\setlength{\belowdisplayshortskip}{.9\parskip}
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% General-purpose enum environment with correct spacing
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\newenvironment{enum}%
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    {\begin{list}{}{%
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        \setlength{\topsep}{.1\parskip}%
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        \setlength{\partopsep}{.1\parskip}%
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        \setlength{\itemsep}{\parskip}%
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        \advance\itemsep by-\parsep}}
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    {\end{list}}
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\def\pre{\begingroup\vskip0pt plus1ex\advance\leftskip by\leftmargin
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\advance\rightskip by\leftmargin}
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\def\post{\vskip0pt plus1ex\endgroup}
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\def\prew{\pre\advance\rightskip by-\leftmargin}
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\def\postw{\post}
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\section{Introduction}
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\label{introduction}
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Nitpick \cite{blanchette-nipkow-2010} is a counterexample generator for
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Isabelle/HOL \cite{isa-tutorial} that is designed to handle formulas
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combining (co)in\-duc\-tive datatypes, (co)in\-duc\-tively defined predicates, and
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quantifiers. It builds on Kodkod \cite{torlak-jackson-2007}, a highly optimized
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first-order relational model finder developed by the Software Design Group at
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MIT. It is conceptually similar to Refute \cite{weber-2008}, from which it
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borrows many ideas and code fragments, but it benefits from Kodkod's
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optimizations and a new encoding scheme. The name Nitpick is shamelessly
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appropriated from a now retired Alloy precursor.
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Nitpick is easy to use---you simply enter \textbf{nitpick} after a putative
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theorem and wait a few seconds. Nonetheless, there are situations where knowing
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how it works under the hood and how it reacts to various options helps
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increase the test coverage. This manual also explains how to install the tool on
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your workstation. Should the motivation fail you, think of the many hours of
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hard work Nitpick will save you. Proving non-theorems is \textsl{hard work}.
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Another common use of Nitpick is to find out whether the axioms of a locale are
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satisfiable, while the locale is being developed. To check this, it suffices to
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write
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\prew
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\textbf{lemma}~``$\textit{False}$'' \\
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\textbf{nitpick}~[\textit{show\_all}]
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\postw
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after the locale's \textbf{begin} keyword. To falsify \textit{False}, Nitpick
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must find a model for the axioms. If it finds no model, we have an indication
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that the axioms might be unsatisfiable.
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You can also invoke Nitpick from the ``Commands'' submenu of the
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``Isabelle'' menu in Proof General or by pressing the Emacs key sequence C-c C-a
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C-n. This is equivalent to entering the \textbf{nitpick} command with no
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arguments in the theory text.
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Nitpick requires the Kodkodi package for Isabelle as well as a Java 1.5 virtual
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machine called \texttt{java}. The examples presented in this manual can be found
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in Isabelle's \texttt{src/HOL/Nitpick\_Examples/Manual\_Nits.thy} theory.
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Throughout this manual, we will explicitly invoke the \textbf{nitpick} command.
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Nitpick also provides an automatic mode that can be enabled via the ``Auto
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Nitpick'' option from the ``Isabelle'' menu in Proof General. In this mode,
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Nitpick is run on every newly entered theorem. The time limit for Auto Nitpick
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and other automatic tools can be set using the ``Auto Tools Time Limit'' option.
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\newbox\boxA
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\setbox\boxA=\hbox{\texttt{nospam}}
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The known bugs and limitations at the time of writing are listed in
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\S\ref{known-bugs-and-limitations}. Comments and bug reports concerning Nitpick
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or this manual should be directed to
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\texttt{blan{\color{white}nospam}\kern-\wd\boxA{}chette@\allowbreak
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in.\allowbreak tum.\allowbreak de}.
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\vskip2.5\smallskipamount
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\textbf{Acknowledgment.} The author would like to thank Mark Summerfield for
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suggesting several textual improvements.
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% and Perry James for reporting a typo.
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%\section{Installation}
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%\label{installation}
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%
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%MISSING:
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%
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%  * Nitpick is part of Isabelle/HOL
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%  * but it relies on an external tool called Kodkodi (Kodkod wrapper)
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%  * Two options:
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%    * if you use a prebuilt Isabelle package, Kodkodi is automatically there
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%    * if you work from sources, the latest Kodkodi can be obtained from ...
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%      download it, install it in some directory of your choice (e.g.,
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%      $ISABELLE_HOME/contrib/kodkodi), and add the absolute path to Kodkodi
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%      in your .isabelle/etc/components file
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%
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%  * If you're not sure, just try the example in the next section
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\section{First Steps}
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\label{first-steps}
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This section introduces Nitpick by presenting small examples. If possible, you
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should try out the examples on your workstation. Your theory file should start
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as follows:
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\prew
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\textbf{theory}~\textit{Scratch} \\
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\textbf{imports}~\textit{Main~Quotient\_Product~RealDef} \\
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\textbf{begin}
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\postw
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The results presented here were obtained using the JNI (Java Native Interface)
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version of MiniSat and with multithreading disabled to reduce nondeterminism.
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This was done by adding the line
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\prew
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\textbf{nitpick\_params} [\textit{sat\_solver}~= \textit{MiniSat\_JNI}, \,\textit{max\_threads}~= 1]
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\postw
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after the \textbf{begin} keyword. The JNI version of MiniSat is bundled with
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Kodkodi and is precompiled for the major platforms. Other SAT solvers can also
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be installed, as explained in \S\ref{optimizations}. If you have already
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configured SAT solvers in Isabelle (e.g., for Refute), these will also be
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available to Nitpick.
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\subsection{Propositional Logic}
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\label{propositional-logic}
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Let's start with a trivial example from propositional logic:
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\prew
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\textbf{lemma}~``$P \longleftrightarrow Q$'' \\
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\textbf{nitpick}
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\postw
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You should get the following output:
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\prew
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\slshape
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Nitpick found a counterexample: \\[2\smallskipamount]
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\hbox{}\qquad Free variables: \nopagebreak \\
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\hbox{}\qquad\qquad $P = \textit{True}$ \\
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\hbox{}\qquad\qquad $Q = \textit{False}$
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\postw
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%FIXME: If you get the output:...
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%Then do such-and-such.
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Nitpick can also be invoked on individual subgoals, as in the example below:
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\prew
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\textbf{apply}~\textit{auto} \\[2\smallskipamount]
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{\slshape goal (2 subgoals): \\
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\phantom{0}1. $P\,\Longrightarrow\, Q$ \\
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\phantom{0}2. $Q\,\Longrightarrow\, P$} \\[2\smallskipamount]
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\textbf{nitpick}~1 \\[2\smallskipamount]
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{\slshape Nitpick found a counterexample: \\[2\smallskipamount]
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\hbox{}\qquad Free variables: \nopagebreak \\
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\hbox{}\qquad\qquad $P = \textit{True}$ \\
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\hbox{}\qquad\qquad $Q = \textit{False}$} \\[2\smallskipamount]
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\textbf{nitpick}~2 \\[2\smallskipamount]
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{\slshape Nitpick found a counterexample: \\[2\smallskipamount]
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\hbox{}\qquad Free variables: \nopagebreak \\
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\hbox{}\qquad\qquad $P = \textit{False}$ \\
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\hbox{}\qquad\qquad $Q = \textit{True}$} \\[2\smallskipamount]
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\textbf{oops}
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\postw
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\subsection{Type Variables}
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\label{type-variables}
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If you are left unimpressed by the previous example, don't worry. The next
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one is more mind- and computer-boggling:
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\prew
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\textbf{lemma} ``$P~x\,\Longrightarrow\, P~(\textrm{THE}~y.\;P~y)$''
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\postw
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\pagebreak[2] %% TYPESETTING
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The putative lemma involves the definite description operator, {THE}, presented
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in section 5.10.1 of the Isabelle tutorial \cite{isa-tutorial}. The
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operator is defined by the axiom $(\textrm{THE}~x.\; x = a) = a$. The putative
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lemma is merely asserting the indefinite description operator axiom with {THE}
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substituted for {SOME}.
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The free variable $x$ and the bound variable $y$ have type $'a$. For formulas
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containing type variables, Nitpick enumerates the possible domains for each type
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variable, up to a given cardinality (10 by default), looking for a finite
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countermodel:
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\prew
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\textbf{nitpick} [\textit{verbose}] \\[2\smallskipamount]
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\slshape
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Trying 10 scopes: \nopagebreak \\
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\hbox{}\qquad \textit{card}~$'a$~= 1; \\
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\hbox{}\qquad \textit{card}~$'a$~= 2; \\
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\hbox{}\qquad $\qquad\vdots$ \\[.5\smallskipamount]
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\hbox{}\qquad \textit{card}~$'a$~= 10. \\[2\smallskipamount]
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Nitpick found a counterexample for \textit{card} $'a$~= 3: \\[2\smallskipamount]
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\hbox{}\qquad Free variables: \nopagebreak \\
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\hbox{}\qquad\qquad $P = \{a_2,\, a_3\}$ \\
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\hbox{}\qquad\qquad $x = a_3$ \\[2\smallskipamount]
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Total time: 768 ms.
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\postw
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Nitpick found a counterexample in which $'a$ has cardinality 3. (For
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cardinalities 1 and 2, the formula holds.) In the counterexample, the three
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values of type $'a$ are written $a_1$, $a_2$, and $a_3$.
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The message ``Trying $n$ scopes: {\ldots}''\ is shown only if the option
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\textit{verbose} is enabled. You can specify \textit{verbose} each time you
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invoke \textbf{nitpick}, or you can set it globally using the command
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\prew
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\textbf{nitpick\_params} [\textit{verbose}]
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\postw
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This command also displays the current default values for all of the options
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supported by Nitpick. The options are listed in \S\ref{option-reference}.
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\subsection{Constants}
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\label{constants}
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By just looking at Nitpick's output, it might not be clear why the
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counterexample in \S\ref{type-variables} is genuine. Let's invoke Nitpick again,
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this time telling it to show the values of the constants that occur in the
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formula:
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\prew
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\textbf{lemma}~``$P~x\,\Longrightarrow\, P~(\textrm{THE}~y.\;P~y)$'' \\
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\textbf{nitpick}~[\textit{show\_consts}] \\[2\smallskipamount]
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\slshape
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Nitpick found a counterexample for \textit{card} $'a$~= 3: \\[2\smallskipamount]
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\hbox{}\qquad Free variables: \nopagebreak \\
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\hbox{}\qquad\qquad $P = \{a_2,\, a_3\}$ \\
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\hbox{}\qquad\qquad $x = a_3$ \\
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\hbox{}\qquad Constant: \nopagebreak \\
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\hbox{}\qquad\qquad $\hbox{\slshape THE}~y.\;P~y = a_1$
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\postw
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As the result of an optimization, Nitpick directly assigned a value to the
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subterm $\textrm{THE}~y.\;P~y$, rather than to the \textit{The} constant. If we
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disable this optimization by using the command
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\prew
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\textbf{nitpick}~[\textit{dont\_specialize},\, \textit{show\_consts}]
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\postw
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we get \textit{The}:
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\prew
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\slshape Constant: \nopagebreak \\
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\hbox{}\qquad $\mathit{The} = \undef{}
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    (\!\begin{aligned}[t]%
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    & \{a_1, a_2, a_3\} := a_3,\> \{a_1, a_2\} := a_3,\> \{a_1, a_3\} := a_3, \\[-2pt] %% TYPESETTING
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    & \{a_1\} := a_1,\> \{a_2, a_3\} := a_1,\> \{a_2\} := a_2, \\[-2pt]
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    & \{a_3\} := a_3,\> \{\} := a_3)\end{aligned}$
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\postw
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Notice that $\textit{The}~(\lambda y.\;P~y) = \textit{The}~\{a_2, a_3\} = a_1$,
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just like before.\footnote{The Isabelle/HOL notation $f(x :=
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y)$ denotes the function that maps $x$ to $y$ and that otherwise behaves like
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$f$.}
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Our misadventures with THE suggest adding `$\exists!x{.}$' (``there exists a
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unique $x$ such that'') at the front of our putative lemma's assumption:
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\prew
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\textbf{lemma}~``$\exists {!}x.\; P~x\,\Longrightarrow\, P~(\textrm{THE}~y.\;P~y)$''
blanchet@33191
   331
\postw
blanchet@33191
   332
blanchet@33191
   333
The fix appears to work:
blanchet@33191
   334
blanchet@33191
   335
\prew
blanchet@33191
   336
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   337
\slshape Nitpick found no counterexample.
blanchet@33191
   338
\postw
blanchet@33191
   339
blanchet@33191
   340
We can further increase our confidence in the formula by exhausting all
blanchet@33191
   341
cardinalities up to 50:
blanchet@33191
   342
blanchet@33191
   343
\prew
blanchet@33191
   344
\textbf{nitpick} [\textit{card} $'a$~= 1--50]\footnote{The symbol `--'
blanchet@33191
   345
can be entered as \texttt{-} (hyphen) or
blanchet@33191
   346
\texttt{\char`\\\char`\<midarrow\char`\>}.} \\[2\smallskipamount]
blanchet@33191
   347
\slshape Nitpick found no counterexample.
blanchet@33191
   348
\postw
blanchet@33191
   349
blanchet@38181
   350
Let's see if Sledgehammer can find a proof:
blanchet@33191
   351
blanchet@33191
   352
\prew
blanchet@33191
   353
\textbf{sledgehammer} \\[2\smallskipamount]
blanchet@33191
   354
{\slshape Sledgehammer: external prover ``$e$'' for subgoal 1: \\
blanchet@33191
   355
$\exists{!}x.\; P~x\,\Longrightarrow\, P~(\hbox{\slshape THE}~y.\; P~y)$ \\
blanchet@33191
   356
Try this command: \textrm{apply}~(\textit{metis~the\_equality})} \\[2\smallskipamount]
blanchet@33191
   357
\textbf{apply}~(\textit{metis~the\_equality\/}) \nopagebreak \\[2\smallskipamount]
blanchet@33191
   358
{\slshape No subgoals!}% \\[2\smallskipamount]
blanchet@33191
   359
%\textbf{done}
blanchet@33191
   360
\postw
blanchet@33191
   361
blanchet@33191
   362
This must be our lucky day.
blanchet@33191
   363
blanchet@33191
   364
\subsection{Skolemization}
blanchet@33191
   365
\label{skolemization}
blanchet@33191
   366
blanchet@33191
   367
Are all invertible functions onto? Let's find out:
blanchet@33191
   368
blanchet@33191
   369
\prew
blanchet@33191
   370
\textbf{lemma} ``$\exists g.\; \forall x.~g~(f~x) = x
blanchet@33191
   371
 \,\Longrightarrow\, \forall y.\; \exists x.~y = f~x$'' \\
blanchet@33191
   372
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   373
\slshape
blanchet@33191
   374
Nitpick found a counterexample for \textit{card} $'a$~= 2 and \textit{card} $'b$~=~1: \\[2\smallskipamount]
blanchet@33191
   375
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   376
\hbox{}\qquad\qquad $f = \undef{}(b_1 := a_1)$ \\
blanchet@33191
   377
\hbox{}\qquad Skolem constants: \nopagebreak \\
blanchet@33191
   378
\hbox{}\qquad\qquad $g = \undef{}(a_1 := b_1,\> a_2 := b_1)$ \\
blanchet@33191
   379
\hbox{}\qquad\qquad $y = a_2$
blanchet@33191
   380
\postw
blanchet@33191
   381
blanchet@33191
   382
Although $f$ is the only free variable occurring in the formula, Nitpick also
blanchet@33191
   383
displays values for the bound variables $g$ and $y$. These values are available
blanchet@33191
   384
to Nitpick because it performs skolemization as a preprocessing step.
blanchet@33191
   385
blanchet@33191
   386
In the previous example, skolemization only affected the outermost quantifiers.
blanchet@33191
   387
This is not always the case, as illustrated below:
blanchet@33191
   388
blanchet@33191
   389
\prew
blanchet@33191
   390
\textbf{lemma} ``$\exists x.\; \forall f.\; f~x = x$'' \\
blanchet@33191
   391
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   392
\slshape
blanchet@33191
   393
Nitpick found a counterexample for \textit{card} $'a$~= 2: \\[2\smallskipamount]
blanchet@33191
   394
\hbox{}\qquad Skolem constant: \nopagebreak \\
blanchet@33191
   395
\hbox{}\qquad\qquad $\lambda x.\; f =
blanchet@33191
   396
    \undef{}(\!\begin{aligned}[t]
blanchet@33191
   397
    & a_1 := \undef{}(a_1 := a_2,\> a_2 := a_1), \\[-2pt]
blanchet@33191
   398
    & a_2 := \undef{}(a_1 := a_1,\> a_2 := a_1))\end{aligned}$
blanchet@33191
   399
\postw
blanchet@33191
   400
blanchet@33191
   401
The variable $f$ is bound within the scope of $x$; therefore, $f$ depends on
blanchet@33191
   402
$x$, as suggested by the notation $\lambda x.\,f$. If $x = a_1$, then $f$ is the
blanchet@33191
   403
function that maps $a_1$ to $a_2$ and vice versa; otherwise, $x = a_2$ and $f$
blanchet@33191
   404
maps both $a_1$ and $a_2$ to $a_1$. In both cases, $f~x \not= x$.
blanchet@33191
   405
blanchet@33191
   406
The source of the Skolem constants is sometimes more obscure:
blanchet@33191
   407
blanchet@33191
   408
\prew
blanchet@33191
   409
\textbf{lemma} ``$\mathit{refl}~r\,\Longrightarrow\, \mathit{sym}~r$'' \\
blanchet@33191
   410
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   411
\slshape
blanchet@33191
   412
Nitpick found a counterexample for \textit{card} $'a$~= 2: \\[2\smallskipamount]
blanchet@33191
   413
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   414
\hbox{}\qquad\qquad $r = \{(a_1, a_1),\, (a_2, a_1),\, (a_2, a_2)\}$ \\
blanchet@33191
   415
\hbox{}\qquad Skolem constants: \nopagebreak \\
blanchet@33191
   416
\hbox{}\qquad\qquad $\mathit{sym}.x = a_2$ \\
blanchet@33191
   417
\hbox{}\qquad\qquad $\mathit{sym}.y = a_1$
blanchet@33191
   418
\postw
blanchet@33191
   419
blanchet@33191
   420
What happened here is that Nitpick expanded the \textit{sym} constant to its
blanchet@33191
   421
definition:
blanchet@33191
   422
blanchet@33191
   423
\prew
blanchet@33191
   424
$\mathit{sym}~r \,\equiv\,
blanchet@33191
   425
 \forall x\> y.\,\> (x, y) \in r \longrightarrow (y, x) \in r.$
blanchet@33191
   426
\postw
blanchet@33191
   427
blanchet@33191
   428
As their names suggest, the Skolem constants $\mathit{sym}.x$ and
blanchet@33191
   429
$\mathit{sym}.y$ are simply the bound variables $x$ and $y$
blanchet@33191
   430
from \textit{sym}'s definition.
blanchet@33191
   431
blanchet@33191
   432
\subsection{Natural Numbers and Integers}
blanchet@33191
   433
\label{natural-numbers-and-integers}
blanchet@33191
   434
blanchet@33191
   435
Because of the axiom of infinity, the type \textit{nat} does not admit any
blanchet@34124
   436
finite models. To deal with this, Nitpick's approach is to consider finite
blanchet@34124
   437
subsets $N$ of \textit{nat} and maps all numbers $\notin N$ to the undefined
blanchet@34124
   438
value (displayed as `$\unk$'). The type \textit{int} is handled similarly.
blanchet@34124
   439
Internally, undefined values lead to a three-valued logic.
blanchet@33191
   440
blanchet@35284
   441
Here is an example involving \textit{int\/}:
blanchet@33191
   442
blanchet@33191
   443
\prew
blanchet@33191
   444
\textbf{lemma} ``$\lbrakk i \le j;\> n \le (m{\Colon}\mathit{int})\rbrakk \,\Longrightarrow\, i * n + j * m \le i * m + j * n$'' \\
blanchet@33191
   445
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   446
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   447
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
   448
\hbox{}\qquad\qquad $i = 0$ \\
blanchet@33191
   449
\hbox{}\qquad\qquad $j = 1$ \\
blanchet@33191
   450
\hbox{}\qquad\qquad $m = 1$ \\
blanchet@33191
   451
\hbox{}\qquad\qquad $n = 0$
blanchet@33191
   452
\postw
blanchet@33191
   453
blanchet@34124
   454
Internally, Nitpick uses either a unary or a binary representation of numbers.
blanchet@34124
   455
The unary representation is more efficient but only suitable for numbers very
blanchet@34124
   456
close to zero. By default, Nitpick attempts to choose the more appropriate
blanchet@34124
   457
encoding by inspecting the formula at hand. This behavior can be overridden by
blanchet@34124
   458
passing either \textit{unary\_ints} or \textit{binary\_ints} as option. For
blanchet@34124
   459
binary notation, the number of bits to use can be specified using
blanchet@34124
   460
the \textit{bits} option. For example:
blanchet@34124
   461
blanchet@34124
   462
\prew
blanchet@34124
   463
\textbf{nitpick} [\textit{binary\_ints}, \textit{bits}${} = 16$]
blanchet@34124
   464
\postw
blanchet@34124
   465
blanchet@33191
   466
With infinite types, we don't always have the luxury of a genuine counterexample
blanchet@33191
   467
and must often content ourselves with a potential one. The tedious task of
blanchet@33191
   468
finding out whether the potential counterexample is in fact genuine can be
blanchet@34124
   469
outsourced to \textit{auto} by passing \textit{check\_potential}. For example:
blanchet@33191
   470
blanchet@33191
   471
\prew
blanchet@33191
   472
\textbf{lemma} ``$\forall n.\; \textit{Suc}~n \mathbin{\not=} n \,\Longrightarrow\, P$'' \\
blanchet@35712
   473
\textbf{nitpick} [\textit{card~nat}~= 50, \textit{check\_potential}] \\[2\smallskipamount]
blanchet@35385
   474
\slshape Warning: The conjecture either trivially holds for the given scopes or lies outside Nitpick's supported
blanchet@35185
   475
fragment. Only potential counterexamples may be found. \\[2\smallskipamount]
blanchet@35185
   476
Nitpick found a potential counterexample: \\[2\smallskipamount]
blanchet@33191
   477
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   478
\hbox{}\qquad\qquad $P = \textit{False}$ \\[2\smallskipamount]
blanchet@33191
   479
Confirmation by ``\textit{auto}'': The above counterexample is genuine.
blanchet@33191
   480
\postw
blanchet@33191
   481
blanchet@33191
   482
You might wonder why the counterexample is first reported as potential. The root
blanchet@33191
   483
of the problem is that the bound variable in $\forall n.\; \textit{Suc}~n
blanchet@33191
   484
\mathbin{\not=} n$ ranges over an infinite type. If Nitpick finds an $n$ such
blanchet@33191
   485
that $\textit{Suc}~n \mathbin{=} n$, it evaluates the assumption to
blanchet@33191
   486
\textit{False}; but otherwise, it does not know anything about values of $n \ge
blanchet@33191
   487
\textit{card~nat}$ and must therefore evaluate the assumption to $\unk$, not
blanchet@33191
   488
\textit{True}. Since the assumption can never be satisfied, the putative lemma
blanchet@33191
   489
can never be falsified.
blanchet@33191
   490
blanchet@33191
   491
Incidentally, if you distrust the so-called genuine counterexamples, you can
blanchet@33191
   492
enable \textit{check\_\allowbreak genuine} to verify them as well. However, be
blanchet@34124
   493
aware that \textit{auto} will usually fail to prove that the counterexample is
blanchet@33191
   494
genuine or spurious.
blanchet@33191
   495
blanchet@33191
   496
Some conjectures involving elementary number theory make Nitpick look like a
blanchet@33191
   497
giant with feet of clay:
blanchet@33191
   498
blanchet@33191
   499
\prew
blanchet@33191
   500
\textbf{lemma} ``$P~\textit{Suc}$'' \\
blanchet@35309
   501
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   502
\slshape
blanchet@33191
   503
Nitpick found no counterexample.
blanchet@33191
   504
\postw
blanchet@33191
   505
blanchet@34124
   506
On any finite set $N$, \textit{Suc} is a partial function; for example, if $N =
blanchet@34124
   507
\{0, 1, \ldots, k\}$, then \textit{Suc} is $\{0 \mapsto 1,\, 1 \mapsto 2,\,
blanchet@34124
   508
\ldots,\, k \mapsto \unk\}$, which evaluates to $\unk$ when passed as
blanchet@34124
   509
argument to $P$. As a result, $P~\textit{Suc}$ is always $\unk$. The next
blanchet@34124
   510
example is similar:
blanchet@33191
   511
blanchet@33191
   512
\prew
blanchet@33191
   513
\textbf{lemma} ``$P~(\textit{op}~{+}\Colon
blanchet@33191
   514
\textit{nat}\mathbin{\Rightarrow}\textit{nat}\mathbin{\Rightarrow}\textit{nat})$'' \\
blanchet@33191
   515
\textbf{nitpick} [\textit{card nat} = 1] \\[2\smallskipamount]
blanchet@33191
   516
{\slshape Nitpick found a counterexample:} \\[2\smallskipamount]
blanchet@33191
   517
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   518
\hbox{}\qquad\qquad $P = \{\}$ \\[2\smallskipamount]
blanchet@33191
   519
\textbf{nitpick} [\textit{card nat} = 2] \\[2\smallskipamount]
blanchet@33191
   520
{\slshape Nitpick found no counterexample.}
blanchet@33191
   521
\postw
blanchet@33191
   522
blanchet@33191
   523
The problem here is that \textit{op}~+ is total when \textit{nat} is taken to be
blanchet@33191
   524
$\{0\}$ but becomes partial as soon as we add $1$, because $1 + 1 \notin \{0,
blanchet@33191
   525
1\}$.
blanchet@33191
   526
blanchet@33191
   527
Because numbers are infinite and are approximated using a three-valued logic,
blanchet@33191
   528
there is usually no need to systematically enumerate domain sizes. If Nitpick
blanchet@33191
   529
cannot find a genuine counterexample for \textit{card~nat}~= $k$, it is very
blanchet@33191
   530
unlikely that one could be found for smaller domains. (The $P~(\textit{op}~{+})$
blanchet@33191
   531
example above is an exception to this principle.) Nitpick nonetheless enumerates
blanchet@38181
   532
all cardinalities from 1 to 10 for \textit{nat}, mainly because smaller
blanchet@33191
   533
cardinalities are fast to handle and give rise to simpler counterexamples. This
blanchet@33191
   534
is explained in more detail in \S\ref{scope-monotonicity}.
blanchet@33191
   535
blanchet@33191
   536
\subsection{Inductive Datatypes}
blanchet@33191
   537
\label{inductive-datatypes}
blanchet@33191
   538
blanchet@33191
   539
Like natural numbers and integers, inductive datatypes with recursive
blanchet@33191
   540
constructors admit no finite models and must be approximated by a subterm-closed
blanchet@33191
   541
subset. For example, using a cardinality of 10 for ${'}a~\textit{list}$,
blanchet@33191
   542
Nitpick looks for all counterexamples that can be built using at most 10
blanchet@33191
   543
different lists.
blanchet@33191
   544
blanchet@33191
   545
Let's see with an example involving \textit{hd} (which returns the first element
blanchet@33191
   546
of a list) and $@$ (which concatenates two lists):
blanchet@33191
   547
blanchet@33191
   548
\prew
blanchet@33191
   549
\textbf{lemma} ``$\textit{hd}~(\textit{xs} \mathbin{@} [y, y]) = \textit{hd}~\textit{xs}$'' \\
blanchet@33191
   550
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   551
\slshape Nitpick found a counterexample for \textit{card} $'a$~= 3: \\[2\smallskipamount]
blanchet@33191
   552
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
   553
\hbox{}\qquad\qquad $\textit{xs} = []$ \\
blanchet@35078
   554
\hbox{}\qquad\qquad $\textit{y} = a_1$
blanchet@33191
   555
\postw
blanchet@33191
   556
blanchet@33191
   557
To see why the counterexample is genuine, we enable \textit{show\_consts}
blanchet@33191
   558
and \textit{show\_\allowbreak datatypes}:
blanchet@33191
   559
blanchet@33191
   560
\prew
blanchet@33191
   561
{\slshape Datatype:} \\
blanchet@35078
   562
\hbox{}\qquad $'a$~\textit{list}~= $\{[],\, [a_1],\, [a_1, a_1],\, \unr\}$ \\
blanchet@33191
   563
{\slshape Constants:} \\
blanchet@35078
   564
\hbox{}\qquad $\lambda x_1.\; x_1 \mathbin{@} [y, y] = \undef([] := [a_1, a_1])$ \\
blanchet@35078
   565
\hbox{}\qquad $\textit{hd} = \undef([] := a_2,\> [a_1] := a_1,\> [a_1, a_1] := a_1)$
blanchet@33191
   566
\postw
blanchet@33191
   567
blanchet@33191
   568
Since $\mathit{hd}~[]$ is undefined in the logic, it may be given any value,
blanchet@33191
   569
including $a_2$.
blanchet@33191
   570
blanchet@33191
   571
The second constant, $\lambda x_1.\; x_1 \mathbin{@} [y, y]$, is simply the
blanchet@35078
   572
append operator whose second argument is fixed to be $[y, y]$. Appending $[a_1,
blanchet@35078
   573
a_1]$ to $[a_1]$ would normally give $[a_1, a_1, a_1]$, but this value is not
blanchet@33191
   574
representable in the subset of $'a$~\textit{list} considered by Nitpick, which
blanchet@33191
   575
is shown under the ``Datatype'' heading; hence the result is $\unk$. Similarly,
blanchet@35078
   576
appending $[a_1, a_1]$ to itself gives $\unk$.
blanchet@33191
   577
blanchet@33191
   578
Given \textit{card}~$'a = 3$ and \textit{card}~$'a~\textit{list} = 3$, Nitpick
blanchet@33191
   579
considers the following subsets:
blanchet@33191
   580
blanchet@33191
   581
\kern-.5\smallskipamount %% TYPESETTING
blanchet@33191
   582
blanchet@33191
   583
\prew
blanchet@33191
   584
\begin{multicols}{3}
blanchet@33191
   585
$\{[],\, [a_1],\, [a_2]\}$; \\
blanchet@33191
   586
$\{[],\, [a_1],\, [a_3]\}$; \\
blanchet@33191
   587
$\{[],\, [a_2],\, [a_3]\}$; \\
blanchet@33191
   588
$\{[],\, [a_1],\, [a_1, a_1]\}$; \\
blanchet@33191
   589
$\{[],\, [a_1],\, [a_2, a_1]\}$; \\
blanchet@33191
   590
$\{[],\, [a_1],\, [a_3, a_1]\}$; \\
blanchet@33191
   591
$\{[],\, [a_2],\, [a_1, a_2]\}$; \\
blanchet@33191
   592
$\{[],\, [a_2],\, [a_2, a_2]\}$; \\
blanchet@33191
   593
$\{[],\, [a_2],\, [a_3, a_2]\}$; \\
blanchet@33191
   594
$\{[],\, [a_3],\, [a_1, a_3]\}$; \\
blanchet@33191
   595
$\{[],\, [a_3],\, [a_2, a_3]\}$; \\
blanchet@33191
   596
$\{[],\, [a_3],\, [a_3, a_3]\}$.
blanchet@33191
   597
\end{multicols}
blanchet@33191
   598
\postw
blanchet@33191
   599
blanchet@33191
   600
\kern-2\smallskipamount %% TYPESETTING
blanchet@33191
   601
blanchet@33191
   602
All subterm-closed subsets of $'a~\textit{list}$ consisting of three values
blanchet@33191
   603
are listed and only those. As an example of a non-subterm-closed subset,
blanchet@35078
   604
consider $\mathcal{S} = \{[],\, [a_1],\,\allowbreak [a_1, a_2]\}$, and observe
blanchet@35078
   605
that $[a_1, a_2]$ (i.e., $a_1 \mathbin{\#} [a_2]$) has $[a_2] \notin
blanchet@33191
   606
\mathcal{S}$ as a subterm.
blanchet@33191
   607
blanchet@33191
   608
Here's another m\"ochtegern-lemma that Nitpick can refute without a blink:
blanchet@33191
   609
blanchet@33191
   610
\prew
blanchet@33191
   611
\textbf{lemma} ``$\lbrakk \textit{length}~\textit{xs} = 1;\> \textit{length}~\textit{ys} = 1
blanchet@33191
   612
\rbrakk \,\Longrightarrow\, \textit{xs} = \textit{ys}$''
blanchet@33191
   613
\\
blanchet@33191
   614
\textbf{nitpick} [\textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@33191
   615
\slshape Nitpick found a counterexample for \textit{card} $'a$~= 3: \\[2\smallskipamount]
blanchet@33191
   616
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
   617
\hbox{}\qquad\qquad $\textit{xs} = [a_1]$ \\
blanchet@35078
   618
\hbox{}\qquad\qquad $\textit{ys} = [a_2]$ \\
blanchet@33191
   619
\hbox{}\qquad Datatypes: \\
blanchet@33191
   620
\hbox{}\qquad\qquad $\textit{nat} = \{0,\, 1,\, 2,\, \unr\}$ \\
blanchet@35078
   621
\hbox{}\qquad\qquad $'a$~\textit{list} = $\{[],\, [a_1],\, [a_2],\, \unr\}$
blanchet@33191
   622
\postw
blanchet@33191
   623
blanchet@33191
   624
Because datatypes are approximated using a three-valued logic, there is usually
blanchet@33191
   625
no need to systematically enumerate cardinalities: If Nitpick cannot find a
blanchet@33191
   626
genuine counterexample for \textit{card}~$'a~\textit{list}$~= 10, it is very
blanchet@33191
   627
unlikely that one could be found for smaller cardinalities.
blanchet@33191
   628
blanchet@35284
   629
\subsection{Typedefs, Quotient Types, Records, Rationals, and Reals}
blanchet@35712
   630
\label{typedefs-quotient-types-records-rationals-and-reals}
blanchet@33191
   631
blanchet@33191
   632
Nitpick generally treats types declared using \textbf{typedef} as datatypes
blanchet@33191
   633
whose single constructor is the corresponding \textit{Abs\_\kern.1ex} function.
blanchet@33191
   634
For example:
blanchet@33191
   635
blanchet@33191
   636
\prew
blanchet@33191
   637
\textbf{typedef}~\textit{three} = ``$\{0\Colon\textit{nat},\, 1,\, 2\}$'' \\
blanchet@33191
   638
\textbf{by}~\textit{blast} \\[2\smallskipamount]
blanchet@33191
   639
\textbf{definition}~$A \mathbin{\Colon} \textit{three}$ \textbf{where} ``\kern-.1em$A \,\equiv\, \textit{Abs\_\allowbreak three}~0$'' \\
blanchet@33191
   640
\textbf{definition}~$B \mathbin{\Colon} \textit{three}$ \textbf{where} ``$B \,\equiv\, \textit{Abs\_three}~1$'' \\
blanchet@33191
   641
\textbf{definition}~$C \mathbin{\Colon} \textit{three}$ \textbf{where} ``$C \,\equiv\, \textit{Abs\_three}~2$'' \\[2\smallskipamount]
blanchet@33191
   642
\textbf{lemma} ``$\lbrakk P~A;\> P~B\rbrakk \,\Longrightarrow\, P~x$'' \\
blanchet@33191
   643
\textbf{nitpick} [\textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@33191
   644
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   645
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
   646
\hbox{}\qquad\qquad $P = \{\Abs{0},\, \Abs{1}\}$ \\
blanchet@33191
   647
\hbox{}\qquad\qquad $x = \Abs{2}$ \\
blanchet@33191
   648
\hbox{}\qquad Datatypes: \\
blanchet@33191
   649
\hbox{}\qquad\qquad $\textit{nat} = \{0,\, 1,\, 2,\, \unr\}$ \\
blanchet@35078
   650
\hbox{}\qquad\qquad $\textit{three} = \{\Abs{0},\, \Abs{1},\, \Abs{2},\, \unr\}$
blanchet@33191
   651
\postw
blanchet@33191
   652
blanchet@33191
   653
In the output above, $\Abs{n}$ abbreviates $\textit{Abs\_three}~n$.
blanchet@33191
   654
blanchet@35284
   655
Quotient types are handled in much the same way. The following fragment defines
blanchet@35284
   656
the integer type \textit{my\_int} by encoding the integer $x$ by a pair of
blanchet@35284
   657
natural numbers $(m, n)$ such that $x + n = m$:
blanchet@35284
   658
blanchet@35284
   659
\prew
blanchet@35284
   660
\textbf{fun} \textit{my\_int\_rel} \textbf{where} \\
blanchet@35284
   661
``$\textit{my\_int\_rel}~(x,\, y)~(u,\, v) = (x + v = u + y)$'' \\[2\smallskipamount]
blanchet@35284
   662
%
blanchet@35284
   663
\textbf{quotient\_type}~\textit{my\_int} = ``$\textit{nat} \times \textit{nat\/}$''$\;{/}\;$\textit{my\_int\_rel} \\
blanchet@35284
   664
\textbf{by}~(\textit{auto simp add\/}:\ \textit{equivp\_def expand\_fun\_eq}) \\[2\smallskipamount]
blanchet@35284
   665
%
blanchet@35284
   666
\textbf{definition}~\textit{add\_raw}~\textbf{where} \\
blanchet@35284
   667
``$\textit{add\_raw} \,\equiv\, \lambda(x,\, y)~(u,\, v).\; (x + (u\Colon\textit{nat}), y + (v\Colon\textit{nat}))$'' \\[2\smallskipamount]
blanchet@35284
   668
%
blanchet@35284
   669
\textbf{quotient\_definition} ``$\textit{add\/}\Colon\textit{my\_int} \Rightarrow \textit{my\_int} \Rightarrow \textit{my\_int\/}$'' \textbf{is} \textit{add\_raw} \\[2\smallskipamount]
blanchet@35284
   670
%
blanchet@35284
   671
\textbf{lemma} ``$\textit{add}~x~y = \textit{add}~x~x$'' \\
blanchet@35284
   672
\textbf{nitpick} [\textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@35284
   673
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@35284
   674
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35284
   675
\hbox{}\qquad\qquad $x = \Abs{(0,\, 0)}$ \\
blanchet@35284
   676
\hbox{}\qquad\qquad $y = \Abs{(1,\, 0)}$ \\
blanchet@35284
   677
\hbox{}\qquad Datatypes: \\
blanchet@35284
   678
\hbox{}\qquad\qquad $\textit{nat} = \{0,\, 1,\, \unr\}$ \\
blanchet@35665
   679
\hbox{}\qquad\qquad $\textit{nat} \times \textit{nat}~[\textsl{boxed\/}] = \{(0,\, 0),\> (1,\, 0),\> \unr\}$ \\
blanchet@35284
   680
\hbox{}\qquad\qquad $\textit{my\_int} = \{\Abs{(0,\, 0)},\> \Abs{(1,\, 0)},\> \unr\}$
blanchet@35284
   681
\postw
blanchet@35284
   682
blanchet@35284
   683
In the counterexample, $\Abs{(0,\, 0)}$ and $\Abs{(1,\, 0)}$ represent the
blanchet@35284
   684
integers $0$ and $1$, respectively. Other representants would have been
blanchet@35712
   685
possible---e.g., $\Abs{(5,\, 5)}$ and $\Abs{(12,\, 11)}$. If we are going to
blanchet@35712
   686
use \textit{my\_int} extensively, it pays off to install a term postprocessor
blanchet@35712
   687
that converts the pair notation to the standard mathematical notation:
blanchet@35712
   688
blanchet@35712
   689
\prew
blanchet@35712
   690
$\textbf{ML}~\,\{{*} \\
blanchet@35712
   691
\!\begin{aligned}[t]
blanchet@35712
   692
%& ({*}~\,\textit{Proof.context} \rightarrow \textit{string} \rightarrow (\textit{typ} \rightarrow \textit{term~list\/}) \rightarrow \textit{typ} \rightarrow \textit{term} \\[-2pt]
blanchet@35712
   693
%& \phantom{(*}~\,{\rightarrow}\;\textit{term}~\,{*}) \\[-2pt]
blanchet@35712
   694
& \textbf{fun}\,~\textit{my\_int\_postproc}~\_~\_~\_~T~(\textit{Const}~\_~\$~(\textit{Const}~\_~\$~\textit{t1}~\$~\textit{t2\/})) = {} \\[-2pt]
blanchet@35712
   695
& \phantom{fun}\,~\textit{HOLogic.mk\_number}~T~(\textit{snd}~(\textit{HOLogic.dest\_number~t1}) \\[-2pt]
blanchet@35712
   696
& \phantom{fun\,~\textit{HOLogic.mk\_number}~T~(}{-}~\textit{snd}~(\textit{HOLogic.dest\_number~t2\/})) \\[-2pt]
blanchet@35712
   697
& \phantom{fun}\!{\mid}\,~\textit{my\_int\_postproc}~\_~\_~\_~\_~t = t \\[-2pt]
blanchet@35712
   698
{*}\}\end{aligned}$ \\[2\smallskipamount]
blanchet@38284
   699
$\textbf{declaration}~\,\{{*} \\
blanchet@35712
   700
\!\begin{aligned}[t]
blanchet@38284
   701
& \textit{Nitpick\_Model.register\_term\_postprocessor}~\!\begin{aligned}[t]
blanchet@38241
   702
  & @\{\textrm{typ}~\textit{my\_int}\} \\[-2pt]
blanchet@38241
   703
  & \textit{my\_int\_postproc}\end{aligned} \\[-2pt]
blanchet@35712
   704
{*}\}\end{aligned}$
blanchet@35712
   705
\postw
blanchet@35284
   706
blanchet@35284
   707
Records are also handled as datatypes with a single constructor:
blanchet@33191
   708
blanchet@33191
   709
\prew
blanchet@33191
   710
\textbf{record} \textit{point} = \\
blanchet@33191
   711
\hbox{}\quad $\textit{Xcoord} \mathbin{\Colon} \textit{int}$ \\
blanchet@33191
   712
\hbox{}\quad $\textit{Ycoord} \mathbin{\Colon} \textit{int}$ \\[2\smallskipamount]
blanchet@33191
   713
\textbf{lemma} ``$\textit{Xcoord}~(p\Colon\textit{point}) = \textit{Xcoord}~(q\Colon\textit{point})$'' \\
blanchet@33191
   714
\textbf{nitpick} [\textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@33191
   715
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   716
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
   717
\hbox{}\qquad\qquad $p = \lparr\textit{Xcoord} = 1,\> \textit{Ycoord} = 1\rparr$ \\
blanchet@35078
   718
\hbox{}\qquad\qquad $q = \lparr\textit{Xcoord} = 0,\> \textit{Ycoord} = 0\rparr$ \\
blanchet@33191
   719
\hbox{}\qquad Datatypes: \\
blanchet@33191
   720
\hbox{}\qquad\qquad $\textit{int} = \{0,\, 1,\, \unr\}$ \\
blanchet@35078
   721
\hbox{}\qquad\qquad $\textit{point} = \{\!\begin{aligned}[t]
blanchet@35078
   722
& \lparr\textit{Xcoord} = 0,\> \textit{Ycoord} = 0\rparr, \\[-2pt] %% TYPESETTING
blanchet@35078
   723
& \lparr\textit{Xcoord} = 1,\> \textit{Ycoord} = 1\rparr,\, \unr\}\end{aligned}$
blanchet@33191
   724
\postw
blanchet@33191
   725
blanchet@33191
   726
Finally, Nitpick provides rudimentary support for rationals and reals using a
blanchet@33191
   727
similar approach:
blanchet@33191
   728
blanchet@33191
   729
\prew
blanchet@33191
   730
\textbf{lemma} ``$4 * x + 3 * (y\Colon\textit{real}) \not= 1/2$'' \\
blanchet@33191
   731
\textbf{nitpick} [\textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@33191
   732
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   733
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
   734
\hbox{}\qquad\qquad $x = 1/2$ \\
blanchet@33191
   735
\hbox{}\qquad\qquad $y = -1/2$ \\
blanchet@33191
   736
\hbox{}\qquad Datatypes: \\
blanchet@33191
   737
\hbox{}\qquad\qquad $\textit{nat} = \{0,\, 1,\, 2,\, 3,\, 4,\, 5,\, 6,\, 7,\, \unr\}$ \\
blanchet@33191
   738
\hbox{}\qquad\qquad $\textit{int} = \{0,\, 1,\, 2,\, 3,\, 4,\, -3,\, -2,\, -1,\, \unr\}$ \\
blanchet@33191
   739
\hbox{}\qquad\qquad $\textit{real} = \{1,\, 0,\, 4,\, -3/2,\, 3,\, 2,\, 1/2,\, -1/2,\, \unr\}$
blanchet@33191
   740
\postw
blanchet@33191
   741
blanchet@33191
   742
\subsection{Inductive and Coinductive Predicates}
blanchet@33191
   743
\label{inductive-and-coinductive-predicates}
blanchet@33191
   744
blanchet@33191
   745
Inductively defined predicates (and sets) are particularly problematic for
blanchet@33191
   746
counterexample generators. They can make Quickcheck~\cite{berghofer-nipkow-2004}
blanchet@33191
   747
loop forever and Refute~\cite{weber-2008} run out of resources. The crux of
blanchet@38176
   748
the problem is that they are defined using a least fixed-point construction.
blanchet@33191
   749
blanchet@33191
   750
Nitpick's philosophy is that not all inductive predicates are equal. Consider
blanchet@33191
   751
the \textit{even} predicate below:
blanchet@33191
   752
blanchet@33191
   753
\prew
blanchet@33191
   754
\textbf{inductive}~\textit{even}~\textbf{where} \\
blanchet@33191
   755
``\textit{even}~0'' $\,\mid$ \\
blanchet@33191
   756
``\textit{even}~$n\,\Longrightarrow\, \textit{even}~(\textit{Suc}~(\textit{Suc}~n))$''
blanchet@33191
   757
\postw
blanchet@33191
   758
blanchet@33191
   759
This predicate enjoys the desirable property of being well-founded, which means
blanchet@33191
   760
that the introduction rules don't give rise to infinite chains of the form
blanchet@33191
   761
blanchet@33191
   762
\prew
blanchet@33191
   763
$\cdots\,\Longrightarrow\, \textit{even}~k''
blanchet@33191
   764
       \,\Longrightarrow\, \textit{even}~k'
blanchet@33191
   765
       \,\Longrightarrow\, \textit{even}~k.$
blanchet@33191
   766
\postw
blanchet@33191
   767
blanchet@33191
   768
For \textit{even}, this is obvious: Any chain ending at $k$ will be of length
blanchet@33191
   769
$k/2 + 1$:
blanchet@33191
   770
blanchet@33191
   771
\prew
blanchet@33191
   772
$\textit{even}~0\,\Longrightarrow\, \textit{even}~2\,\Longrightarrow\, \cdots
blanchet@33191
   773
       \,\Longrightarrow\, \textit{even}~(k - 2)
blanchet@33191
   774
       \,\Longrightarrow\, \textit{even}~k.$
blanchet@33191
   775
\postw
blanchet@33191
   776
blanchet@33191
   777
Wellfoundedness is desirable because it enables Nitpick to use a very efficient
blanchet@38176
   778
fixed-point computation.%
blanchet@33191
   779
\footnote{If an inductive predicate is
blanchet@33191
   780
well-founded, then it has exactly one fixed point, which is simultaneously the
blanchet@33191
   781
least and the greatest fixed point. In these circumstances, the computation of
blanchet@33191
   782
the least fixed point amounts to the computation of an arbitrary fixed point,
blanchet@33191
   783
which can be performed using a straightforward recursive equation.}
blanchet@33191
   784
Moreover, Nitpick can prove wellfoundedness of most well-founded predicates,
blanchet@33191
   785
just as Isabelle's \textbf{function} package usually discharges termination
blanchet@33191
   786
proof obligations automatically.
blanchet@33191
   787
blanchet@33191
   788
Let's try an example:
blanchet@33191
   789
blanchet@33191
   790
\prew
blanchet@33191
   791
\textbf{lemma} ``$\exists n.\; \textit{even}~n \mathrel{\land} \textit{even}~(\textit{Suc}~n)$'' \\
blanchet@35712
   792
\textbf{nitpick}~[\textit{card nat}~= 50, \textit{unary\_ints}, \textit{verbose}] \\[2\smallskipamount]
blanchet@33191
   793
\slshape The inductive predicate ``\textit{even}'' was proved well-founded.
blanchet@33191
   794
Nitpick can compute it efficiently. \\[2\smallskipamount]
blanchet@33191
   795
Trying 1 scope: \\
blanchet@35712
   796
\hbox{}\qquad \textit{card nat}~= 50. \\[2\smallskipamount]
blanchet@35712
   797
Nitpick found a potential counterexample for \textit{card nat}~= 50: \\[2\smallskipamount]
blanchet@33191
   798
\hbox{}\qquad Empty assignment \\[2\smallskipamount]
blanchet@38183
   799
Nitpick could not find a better counterexample. It checked 0 of 1 scope. \\[2\smallskipamount]
blanchet@38183
   800
Total time: 1439 ms.
blanchet@33191
   801
\postw
blanchet@33191
   802
blanchet@33191
   803
No genuine counterexample is possible because Nitpick cannot rule out the
blanchet@35712
   804
existence of a natural number $n \ge 50$ such that both $\textit{even}~n$ and
blanchet@33191
   805
$\textit{even}~(\textit{Suc}~n)$ are true. To help Nitpick, we can bound the
blanchet@33191
   806
existential quantifier:
blanchet@33191
   807
blanchet@33191
   808
\prew
blanchet@35712
   809
\textbf{lemma} ``$\exists n \mathbin{\le} 49.\; \textit{even}~n \mathrel{\land} \textit{even}~(\textit{Suc}~n)$'' \\
blanchet@35712
   810
\textbf{nitpick}~[\textit{card nat}~= 50, \textit{unary\_ints}] \\[2\smallskipamount]
blanchet@33191
   811
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   812
\hbox{}\qquad Empty assignment
blanchet@33191
   813
\postw
blanchet@33191
   814
blanchet@33191
   815
So far we were blessed by the wellfoundedness of \textit{even}. What happens if
blanchet@33191
   816
we use the following definition instead?
blanchet@33191
   817
blanchet@33191
   818
\prew
blanchet@33191
   819
\textbf{inductive} $\textit{even}'$ \textbf{where} \\
blanchet@33191
   820
``$\textit{even}'~(0{\Colon}\textit{nat})$'' $\,\mid$ \\
blanchet@33191
   821
``$\textit{even}'~2$'' $\,\mid$ \\
blanchet@33191
   822
``$\lbrakk\textit{even}'~m;\> \textit{even}'~n\rbrakk \,\Longrightarrow\, \textit{even}'~(m + n)$''
blanchet@33191
   823
\postw
blanchet@33191
   824
blanchet@33191
   825
This definition is not well-founded: From $\textit{even}'~0$ and
blanchet@33191
   826
$\textit{even}'~0$, we can derive that $\textit{even}'~0$. Nonetheless, the
blanchet@33191
   827
predicates $\textit{even}$ and $\textit{even}'$ are equivalent.
blanchet@33191
   828
blanchet@33191
   829
Let's check a property involving $\textit{even}'$. To make up for the
blanchet@33191
   830
foreseeable computational hurdles entailed by non-wellfoundedness, we decrease
blanchet@33191
   831
\textit{nat}'s cardinality to a mere 10:
blanchet@33191
   832
blanchet@33191
   833
\prew
blanchet@33191
   834
\textbf{lemma}~``$\exists n \in \{0, 2, 4, 6, 8\}.\;
blanchet@33191
   835
\lnot\;\textit{even}'~n$'' \\
blanchet@33191
   836
\textbf{nitpick}~[\textit{card nat}~= 10,\, \textit{verbose},\, \textit{show\_consts}] \\[2\smallskipamount]
blanchet@33191
   837
\slshape
blanchet@33191
   838
The inductive predicate ``$\textit{even}'\!$'' could not be proved well-founded.
blanchet@33191
   839
Nitpick might need to unroll it. \\[2\smallskipamount]
blanchet@33191
   840
Trying 6 scopes: \\
blanchet@33191
   841
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 0; \\
blanchet@33191
   842
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 1; \\
blanchet@33191
   843
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 2; \\
blanchet@33191
   844
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 4; \\
blanchet@33191
   845
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 8; \\
blanchet@33191
   846
\hbox{}\qquad \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 9. \\[2\smallskipamount]
blanchet@33191
   847
Nitpick found a counterexample for \textit{card nat}~= 10 and \textit{iter} $\textit{even}'$~= 2: \\[2\smallskipamount]
blanchet@33191
   848
\hbox{}\qquad Constant: \nopagebreak \\
blanchet@33191
   849
\hbox{}\qquad\qquad $\lambda i.\; \textit{even}'$ = $\undef(\!\begin{aligned}[t]
blanchet@33191
   850
& 2 := \{0, 2, 4, 6, 8, 1^\Q, 3^\Q, 5^\Q, 7^\Q, 9^\Q\}, \\[-2pt]
blanchet@33191
   851
& 1 := \{0, 2, 4, 1^\Q, 3^\Q, 5^\Q, 6^\Q, 7^\Q, 8^\Q, 9^\Q\}, \\[-2pt]
blanchet@33191
   852
& 0 := \{0, 2, 1^\Q, 3^\Q, 4^\Q, 5^\Q, 6^\Q, 7^\Q, 8^\Q, 9^\Q\})\end{aligned}$ \\[2\smallskipamount]
blanchet@38183
   853
Total time: 2420 ms.
blanchet@33191
   854
\postw
blanchet@33191
   855
blanchet@33191
   856
Nitpick's output is very instructive. First, it tells us that the predicate is
blanchet@33191
   857
unrolled, meaning that it is computed iteratively from the empty set. Then it
blanchet@33191
   858
lists six scopes specifying different bounds on the numbers of iterations:\ 0,
blanchet@33191
   859
1, 2, 4, 8, and~9.
blanchet@33191
   860
blanchet@33191
   861
The output also shows how each iteration contributes to $\textit{even}'$. The
blanchet@33191
   862
notation $\lambda i.\; \textit{even}'$ indicates that the value of the
blanchet@33191
   863
predicate depends on an iteration counter. Iteration 0 provides the basis
blanchet@33191
   864
elements, $0$ and $2$. Iteration 1 contributes $4$ ($= 2 + 2$). Iteration 2
blanchet@33191
   865
throws $6$ ($= 2 + 4 = 4 + 2$) and $8$ ($= 4 + 4$) into the mix. Further
blanchet@33191
   866
iterations would not contribute any new elements.
blanchet@33191
   867
blanchet@33191
   868
Some values are marked with superscripted question
blanchet@33191
   869
marks~(`\lower.2ex\hbox{$^\Q$}'). These are the elements for which the
blanchet@33191
   870
predicate evaluates to $\unk$. Thus, $\textit{even}'$ evaluates to either
blanchet@33191
   871
\textit{True} or $\unk$, never \textit{False}.
blanchet@33191
   872
blanchet@38181
   873
When unrolling a predicate, Nitpick tries 0, 1, 2, 4, 8, 12, 16, 20, 24, and 28
blanchet@33191
   874
iterations. However, these numbers are bounded by the cardinality of the
blanchet@33191
   875
predicate's domain. With \textit{card~nat}~= 10, no more than 9 iterations are
blanchet@33191
   876
ever needed to compute the value of a \textit{nat} predicate. You can specify
blanchet@33191
   877
the number of iterations using the \textit{iter} option, as explained in
blanchet@33191
   878
\S\ref{scope-of-search}.
blanchet@33191
   879
blanchet@33191
   880
In the next formula, $\textit{even}'$ occurs both positively and negatively:
blanchet@33191
   881
blanchet@33191
   882
\prew
blanchet@33191
   883
\textbf{lemma} ``$\textit{even}'~(n - 2) \,\Longrightarrow\, \textit{even}'~n$'' \\
blanchet@34124
   884
\textbf{nitpick} [\textit{card nat} = 10, \textit{show\_consts}] \\[2\smallskipamount]
blanchet@33191
   885
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
   886
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   887
\hbox{}\qquad\qquad $n = 1$ \\
blanchet@33191
   888
\hbox{}\qquad Constants: \nopagebreak \\
blanchet@33191
   889
\hbox{}\qquad\qquad $\lambda i.\; \textit{even}'$ = $\undef(\!\begin{aligned}[t]
blanchet@33191
   890
& 0 := \{0, 2, 1^\Q, 3^\Q, 4^\Q, 5^\Q, 6^\Q, 7^\Q, 8^\Q, 9^\Q\})\end{aligned}$  \\
blanchet@33191
   891
\hbox{}\qquad\qquad $\textit{even}' \subseteq \{0, 2, 4, 6, 8, \unr\}$
blanchet@33191
   892
\postw
blanchet@33191
   893
blanchet@33191
   894
Notice the special constraint $\textit{even}' \subseteq \{0,\, 2,\, 4,\, 6,\,
blanchet@33191
   895
8,\, \unr\}$ in the output, whose right-hand side represents an arbitrary
blanchet@33191
   896
fixed point (not necessarily the least one). It is used to falsify
blanchet@33191
   897
$\textit{even}'~n$. In contrast, the unrolled predicate is used to satisfy
blanchet@33191
   898
$\textit{even}'~(n - 2)$.
blanchet@33191
   899
blanchet@33191
   900
Coinductive predicates are handled dually. For example:
blanchet@33191
   901
blanchet@33191
   902
\prew
blanchet@33191
   903
\textbf{coinductive} \textit{nats} \textbf{where} \\
blanchet@33191
   904
``$\textit{nats}~(x\Colon\textit{nat}) \,\Longrightarrow\, \textit{nats}~x$'' \\[2\smallskipamount]
blanchet@33191
   905
\textbf{lemma} ``$\textit{nats} = \{0, 1, 2, 3, 4\}$'' \\
blanchet@33191
   906
\textbf{nitpick}~[\textit{card nat} = 10,\, \textit{show\_consts}] \\[2\smallskipamount]
blanchet@33191
   907
\slshape Nitpick found a counterexample:
blanchet@33191
   908
\\[2\smallskipamount]
blanchet@33191
   909
\hbox{}\qquad Constants: \nopagebreak \\
blanchet@33191
   910
\hbox{}\qquad\qquad $\lambda i.\; \textit{nats} = \undef(0 := \{\!\begin{aligned}[t]
blanchet@33191
   911
& 0^\Q, 1^\Q, 2^\Q, 3^\Q, 4^\Q, 5^\Q, 6^\Q, 7^\Q, 8^\Q, 9^\Q, \\[-2pt]
blanchet@33191
   912
& \unr\})\end{aligned}$ \\
blanchet@33191
   913
\hbox{}\qquad\qquad $nats \supseteq \{9, 5^\Q, 6^\Q, 7^\Q, 8^\Q, \unr\}$
blanchet@33191
   914
\postw
blanchet@33191
   915
blanchet@33191
   916
As a special case, Nitpick uses Kodkod's transitive closure operator to encode
blanchet@33191
   917
negative occurrences of non-well-founded ``linear inductive predicates,'' i.e.,
blanchet@33191
   918
inductive predicates for which each the predicate occurs in at most one
blanchet@33191
   919
assumption of each introduction rule. For example:
blanchet@33191
   920
blanchet@33191
   921
\prew
blanchet@33191
   922
\textbf{inductive} \textit{odd} \textbf{where} \\
blanchet@33191
   923
``$\textit{odd}~1$'' $\,\mid$ \\
blanchet@33191
   924
``$\lbrakk \textit{odd}~m;\>\, \textit{even}~n\rbrakk \,\Longrightarrow\, \textit{odd}~(m + n)$'' \\[2\smallskipamount]
blanchet@33191
   925
\textbf{lemma}~``$\textit{odd}~n \,\Longrightarrow\, \textit{odd}~(n - 2)$'' \\
blanchet@33191
   926
\textbf{nitpick}~[\textit{card nat} = 10,\, \textit{show\_consts}] \\[2\smallskipamount]
blanchet@33191
   927
\slshape Nitpick found a counterexample:
blanchet@33191
   928
\\[2\smallskipamount]
blanchet@33191
   929
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
   930
\hbox{}\qquad\qquad $n = 1$ \\
blanchet@33191
   931
\hbox{}\qquad Constants: \nopagebreak \\
blanchet@33191
   932
\hbox{}\qquad\qquad $\textit{even} = \{0, 2, 4, 6, 8, \unr\}$ \\
blanchet@33191
   933
\hbox{}\qquad\qquad $\textit{odd}_{\textsl{base}} = \{1, \unr\}$ \\
blanchet@33191
   934
\hbox{}\qquad\qquad $\textit{odd}_{\textsl{step}} = \!
blanchet@33191
   935
\!\begin{aligned}[t]
blanchet@33191
   936
  & \{(0, 0), (0, 2), (0, 4), (0, 6), (0, 8), (1, 1), (1, 3), (1, 5), \\[-2pt]
blanchet@33191
   937
  & \phantom{\{} (1, 7), (1, 9), (2, 2), (2, 4), (2, 6), (2, 8), (3, 3),
blanchet@33191
   938
       (3, 5), \\[-2pt]
blanchet@33191
   939
  & \phantom{\{} (3, 7), (3, 9), (4, 4), (4, 6), (4, 8), (5, 5), (5, 7), (5, 9), \\[-2pt]
blanchet@33191
   940
  & \phantom{\{} (6, 6), (6, 8), (7, 7), (7, 9), (8, 8), (9, 9), \unr\}\end{aligned}$ \\
blanchet@33191
   941
\hbox{}\qquad\qquad $\textit{odd} \subseteq \{1, 3, 5, 7, 9, 8^\Q, \unr\}$
blanchet@33191
   942
\postw
blanchet@33191
   943
blanchet@33191
   944
\noindent
blanchet@33191
   945
In the output, $\textit{odd}_{\textrm{base}}$ represents the base elements and
blanchet@33191
   946
$\textit{odd}_{\textrm{step}}$ is a transition relation that computes new
blanchet@33191
   947
elements from known ones. The set $\textit{odd}$ consists of all the values
blanchet@33191
   948
reachable through the reflexive transitive closure of
blanchet@33191
   949
$\textit{odd}_{\textrm{step}}$ starting with any element from
blanchet@33191
   950
$\textit{odd}_{\textrm{base}}$, namely 1, 3, 5, 7, and 9. Using Kodkod's
blanchet@33191
   951
transitive closure to encode linear predicates is normally either more thorough
blanchet@33191
   952
or more efficient than unrolling (depending on the value of \textit{iter}), but
blanchet@33191
   953
for those cases where it isn't you can disable it by passing the
blanchet@33191
   954
\textit{dont\_star\_linear\_preds} option.
blanchet@33191
   955
blanchet@33191
   956
\subsection{Coinductive Datatypes}
blanchet@33191
   957
\label{coinductive-datatypes}
blanchet@33191
   958
blanchet@33191
   959
While Isabelle regrettably lacks a high-level mechanism for defining coinductive
blanchet@35665
   960
datatypes, the \textit{Coinductive\_List} theory from Andreas Lochbihler's
blanchet@35665
   961
\textit{Coinductive} AFP entry \cite{lochbihler-2010} provides a coinductive
blanchet@35665
   962
``lazy list'' datatype, $'a~\textit{llist}$, defined the hard way. Nitpick
blanchet@35665
   963
supports these lazy lists seamlessly and provides a hook, described in
blanchet@33191
   964
\S\ref{registration-of-coinductive-datatypes}, to register custom coinductive
blanchet@33191
   965
datatypes.
blanchet@33191
   966
blanchet@33191
   967
(Co)intuitively, a coinductive datatype is similar to an inductive datatype but
blanchet@33191
   968
allows infinite objects. Thus, the infinite lists $\textit{ps}$ $=$ $[a, a, a,
blanchet@33191
   969
\ldots]$, $\textit{qs}$ $=$ $[a, b, a, b, \ldots]$, and $\textit{rs}$ $=$ $[0,
blanchet@33191
   970
1, 2, 3, \ldots]$ can be defined as lazy lists using the
blanchet@33191
   971
$\textit{LNil}\mathbin{\Colon}{'}a~\textit{llist}$ and
blanchet@33191
   972
$\textit{LCons}\mathbin{\Colon}{'}a \mathbin{\Rightarrow} {'}a~\textit{llist}
blanchet@33191
   973
\mathbin{\Rightarrow} {'}a~\textit{llist}$ constructors.
blanchet@33191
   974
blanchet@33191
   975
Although it is otherwise no friend of infinity, Nitpick can find counterexamples
blanchet@33191
   976
involving cyclic lists such as \textit{ps} and \textit{qs} above as well as
blanchet@33191
   977
finite lists:
blanchet@33191
   978
blanchet@33191
   979
\prew
blanchet@33191
   980
\textbf{lemma} ``$\textit{xs} \not= \textit{LCons}~a~\textit{xs}$'' \\
blanchet@33191
   981
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
   982
\slshape Nitpick found a counterexample for {\itshape card}~$'a$ = 1: \\[2\smallskipamount]
blanchet@33191
   983
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
   984
\hbox{}\qquad\qquad $\textit{a} = a_1$ \\
blanchet@33191
   985
\hbox{}\qquad\qquad $\textit{xs} = \textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega$
blanchet@33191
   986
\postw
blanchet@33191
   987
blanchet@33191
   988
The notation $\textrm{THE}~\omega.\; \omega = t(\omega)$ stands
blanchet@33191
   989
for the infinite term $t(t(t(\ldots)))$. Hence, \textit{xs} is simply the
blanchet@33191
   990
infinite list $[a_1, a_1, a_1, \ldots]$.
blanchet@33191
   991
blanchet@33191
   992
The next example is more interesting:
blanchet@33191
   993
blanchet@33191
   994
\prew
blanchet@33191
   995
\textbf{lemma}~``$\lbrakk\textit{xs} = \textit{LCons}~a~\textit{xs};\>\,
blanchet@33191
   996
\textit{ys} = \textit{iterates}~(\lambda b.\> a)~b\rbrakk \,\Longrightarrow\, \textit{xs} = \textit{ys}$'' \\
blanchet@33191
   997
\textbf{nitpick} [\textit{verbose}] \\[2\smallskipamount]
blanchet@33191
   998
\slshape The type ``\kern1pt$'a$'' passed the monotonicity test. Nitpick might be able to skip
blanchet@33191
   999
some scopes. \\[2\smallskipamount]
blanchet@38181
  1000
Trying 10 scopes: \\
blanchet@35284
  1001
\hbox{}\qquad \textit{card} $'a$~= 1, \textit{card} ``\kern1pt$'a~\textit{list\/}$''~= 1,
blanchet@33191
  1002
and \textit{bisim\_depth}~= 0. \\
blanchet@33191
  1003
\hbox{}\qquad $\qquad\vdots$ \\[.5\smallskipamount]
blanchet@38181
  1004
\hbox{}\qquad \textit{card} $'a$~= 10, \textit{card} ``\kern1pt$'a~\textit{list\/}$''~= 10,
blanchet@38181
  1005
and \textit{bisim\_depth}~= 9. \\[2\smallskipamount]
blanchet@33191
  1006
Nitpick found a counterexample for {\itshape card}~$'a$ = 2,
blanchet@35284
  1007
\textit{card}~``\kern1pt$'a~\textit{list\/}$''~= 2, and \textit{bisim\_\allowbreak
blanchet@33191
  1008
depth}~= 1:
blanchet@33191
  1009
\\[2\smallskipamount]
blanchet@33191
  1010
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
  1011
\hbox{}\qquad\qquad $\textit{a} = a_1$ \\
blanchet@35078
  1012
\hbox{}\qquad\qquad $\textit{b} = a_2$ \\
blanchet@35078
  1013
\hbox{}\qquad\qquad $\textit{xs} = \textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega$ \\
blanchet@35078
  1014
\hbox{}\qquad\qquad $\textit{ys} = \textit{LCons}~a_2~(\textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega)$ \\[2\smallskipamount]
blanchet@38183
  1015
Total time: 1027 ms.
blanchet@33191
  1016
\postw
blanchet@33191
  1017
blanchet@35078
  1018
The lazy list $\textit{xs}$ is simply $[a_1, a_1, a_1, \ldots]$, whereas
blanchet@35078
  1019
$\textit{ys}$ is $[a_2, a_1, a_1, a_1, \ldots]$, i.e., a lasso-shaped list with
blanchet@35078
  1020
$[a_2]$ as its stem and $[a_1]$ as its cycle. In general, the list segment
blanchet@33191
  1021
within the scope of the {THE} binder corresponds to the lasso's cycle, whereas
blanchet@33191
  1022
the segment leading to the binder is the stem.
blanchet@33191
  1023
blanchet@33191
  1024
A salient property of coinductive datatypes is that two objects are considered
blanchet@33191
  1025
equal if and only if they lead to the same observations. For example, the lazy
blanchet@33191
  1026
lists $\textrm{THE}~\omega.\; \omega =
blanchet@33191
  1027
\textit{LCons}~a~(\textit{LCons}~b~\omega)$ and
blanchet@33191
  1028
$\textit{LCons}~a~(\textrm{THE}~\omega.\; \omega =
blanchet@33191
  1029
\textit{LCons}~b~(\textit{LCons}~a~\omega))$ are identical, because both lead
blanchet@33191
  1030
to the sequence of observations $a$, $b$, $a$, $b$, \hbox{\ldots} (or,
blanchet@33191
  1031
equivalently, both encode the infinite list $[a, b, a, b, \ldots]$). This
blanchet@33191
  1032
concept of equality for coinductive datatypes is called bisimulation and is
blanchet@33191
  1033
defined coinductively.
blanchet@33191
  1034
blanchet@33191
  1035
Internally, Nitpick encodes the coinductive bisimilarity predicate as part of
blanchet@33191
  1036
the Kodkod problem to ensure that distinct objects lead to different
blanchet@33191
  1037
observations. This precaution is somewhat expensive and often unnecessary, so it
blanchet@33191
  1038
can be disabled by setting the \textit{bisim\_depth} option to $-1$. The
blanchet@33191
  1039
bisimilarity check is then performed \textsl{after} the counterexample has been
blanchet@33191
  1040
found to ensure correctness. If this after-the-fact check fails, the
blanchet@35695
  1041
counterexample is tagged as ``quasi genuine'' and Nitpick recommends to try
blanchet@33191
  1042
again with \textit{bisim\_depth} set to a nonnegative integer. Disabling the
blanchet@33191
  1043
check for the previous example saves approximately 150~milli\-seconds; the speed
blanchet@33191
  1044
gains can be more significant for larger scopes.
blanchet@33191
  1045
blanchet@33191
  1046
The next formula illustrates the need for bisimilarity (either as a Kodkod
blanchet@33191
  1047
predicate or as an after-the-fact check) to prevent spurious counterexamples:
blanchet@33191
  1048
blanchet@33191
  1049
\prew
blanchet@33191
  1050
\textbf{lemma} ``$\lbrakk xs = \textit{LCons}~a~\textit{xs};\>\, \textit{ys} = \textit{LCons}~a~\textit{ys}\rbrakk
blanchet@33191
  1051
\,\Longrightarrow\, \textit{xs} = \textit{ys}$'' \\
blanchet@34124
  1052
\textbf{nitpick} [\textit{bisim\_depth} = $-1$, \textit{show\_datatypes}] \\[2\smallskipamount]
blanchet@35695
  1053
\slshape Nitpick found a quasi genuine counterexample for $\textit{card}~'a$ = 2: \\[2\smallskipamount]
blanchet@33191
  1054
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
  1055
\hbox{}\qquad\qquad $a = a_1$ \\
blanchet@33191
  1056
\hbox{}\qquad\qquad $\textit{xs} = \textsl{THE}~\omega.\; \omega =
blanchet@35078
  1057
\textit{LCons}~a_1~\omega$ \\
blanchet@35078
  1058
\hbox{}\qquad\qquad $\textit{ys} = \textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega$ \\
blanchet@33191
  1059
\hbox{}\qquad Codatatype:\strut \nopagebreak \\
blanchet@33191
  1060
\hbox{}\qquad\qquad $'a~\textit{llist} =
blanchet@33191
  1061
\{\!\begin{aligned}[t]
blanchet@35078
  1062
  & \textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega, \\[-2pt]
blanchet@35078
  1063
  & \textsl{THE}~\omega.\; \omega = \textit{LCons}~a_1~\omega,\> \unr\}\end{aligned}$
blanchet@33191
  1064
\\[2\smallskipamount]
blanchet@33191
  1065
Try again with ``\textit{bisim\_depth}'' set to a nonnegative value to confirm
blanchet@33191
  1066
that the counterexample is genuine. \\[2\smallskipamount]
blanchet@33191
  1067
{\upshape\textbf{nitpick}} \\[2\smallskipamount]
blanchet@33191
  1068
\slshape Nitpick found no counterexample.
blanchet@33191
  1069
\postw
blanchet@33191
  1070
blanchet@33191
  1071
In the first \textbf{nitpick} invocation, the after-the-fact check discovered 
blanchet@33191
  1072
that the two known elements of type $'a~\textit{llist}$ are bisimilar.
blanchet@33191
  1073
blanchet@33191
  1074
A compromise between leaving out the bisimilarity predicate from the Kodkod
blanchet@33191
  1075
problem and performing the after-the-fact check is to specify a lower
blanchet@33191
  1076
nonnegative \textit{bisim\_depth} value than the default one provided by
blanchet@33191
  1077
Nitpick. In general, a value of $K$ means that Nitpick will require all lists to
blanchet@33191
  1078
be distinguished from each other by their prefixes of length $K$. Be aware that
blanchet@33191
  1079
setting $K$ to a too low value can overconstrain Nitpick, preventing it from
blanchet@33191
  1080
finding any counterexamples.
blanchet@33191
  1081
blanchet@33191
  1082
\subsection{Boxing}
blanchet@33191
  1083
\label{boxing}
blanchet@33191
  1084
blanchet@33191
  1085
Nitpick normally maps function and product types directly to the corresponding
blanchet@33191
  1086
Kodkod concepts. As a consequence, if $'a$ has cardinality 3 and $'b$ has
blanchet@33191
  1087
cardinality 4, then $'a \times {'}b$ has cardinality 12 ($= 4 \times 3$) and $'a
blanchet@33191
  1088
\Rightarrow {'}b$ has cardinality 64 ($= 4^3$). In some circumstances, it pays
blanchet@33191
  1089
off to treat these types in the same way as plain datatypes, by approximating
blanchet@33191
  1090
them by a subset of a given cardinality. This technique is called ``boxing'' and
blanchet@33191
  1091
is particularly useful for functions passed as arguments to other functions, for
blanchet@33191
  1092
high-arity functions, and for large tuples. Under the hood, boxing involves
blanchet@33191
  1093
wrapping occurrences of the types $'a \times {'}b$ and $'a \Rightarrow {'}b$ in
blanchet@33191
  1094
isomorphic datatypes, as can be seen by enabling the \textit{debug} option.
blanchet@33191
  1095
blanchet@33191
  1096
To illustrate boxing, we consider a formalization of $\lambda$-terms represented
blanchet@33191
  1097
using de Bruijn's notation:
blanchet@33191
  1098
blanchet@33191
  1099
\prew
blanchet@33191
  1100
\textbf{datatype} \textit{tm} = \textit{Var}~\textit{nat}~$\mid$~\textit{Lam}~\textit{tm} $\mid$ \textit{App~tm~tm}
blanchet@33191
  1101
\postw
blanchet@33191
  1102
blanchet@33191
  1103
The $\textit{lift}~t~k$ function increments all variables with indices greater
blanchet@33191
  1104
than or equal to $k$ by one:
blanchet@33191
  1105
blanchet@33191
  1106
\prew
blanchet@33191
  1107
\textbf{primrec} \textit{lift} \textbf{where} \\
blanchet@33191
  1108
``$\textit{lift}~(\textit{Var}~j)~k = \textit{Var}~(\textrm{if}~j < k~\textrm{then}~j~\textrm{else}~j + 1)$'' $\mid$ \\
blanchet@33191
  1109
``$\textit{lift}~(\textit{Lam}~t)~k = \textit{Lam}~(\textit{lift}~t~(k + 1))$'' $\mid$ \\
blanchet@33191
  1110
``$\textit{lift}~(\textit{App}~t~u)~k = \textit{App}~(\textit{lift}~t~k)~(\textit{lift}~u~k)$''
blanchet@33191
  1111
\postw
blanchet@33191
  1112
blanchet@33191
  1113
The $\textit{loose}~t~k$ predicate returns \textit{True} if and only if
blanchet@33191
  1114
term $t$ has a loose variable with index $k$ or more:
blanchet@33191
  1115
blanchet@33191
  1116
\prew
blanchet@33191
  1117
\textbf{primrec}~\textit{loose} \textbf{where} \\
blanchet@33191
  1118
``$\textit{loose}~(\textit{Var}~j)~k = (j \ge k)$'' $\mid$ \\
blanchet@33191
  1119
``$\textit{loose}~(\textit{Lam}~t)~k = \textit{loose}~t~(\textit{Suc}~k)$'' $\mid$ \\
blanchet@33191
  1120
``$\textit{loose}~(\textit{App}~t~u)~k = (\textit{loose}~t~k \mathrel{\lor} \textit{loose}~u~k)$''
blanchet@33191
  1121
\postw
blanchet@33191
  1122
blanchet@33191
  1123
Next, the $\textit{subst}~\sigma~t$ function applies the substitution $\sigma$
blanchet@33191
  1124
on $t$:
blanchet@33191
  1125
blanchet@33191
  1126
\prew
blanchet@33191
  1127
\textbf{primrec}~\textit{subst} \textbf{where} \\
blanchet@33191
  1128
``$\textit{subst}~\sigma~(\textit{Var}~j) = \sigma~j$'' $\mid$ \\
blanchet@33191
  1129
``$\textit{subst}~\sigma~(\textit{Lam}~t) = {}$\phantom{''} \\
blanchet@33191
  1130
\phantom{``}$\textit{Lam}~(\textit{subst}~(\lambda n.\> \textrm{case}~n~\textrm{of}~0 \Rightarrow \textit{Var}~0 \mid \textit{Suc}~m \Rightarrow \textit{lift}~(\sigma~m)~1)~t)$'' $\mid$ \\
blanchet@33191
  1131
``$\textit{subst}~\sigma~(\textit{App}~t~u) = \textit{App}~(\textit{subst}~\sigma~t)~(\textit{subst}~\sigma~u)$''
blanchet@33191
  1132
\postw
blanchet@33191
  1133
blanchet@33191
  1134
A substitution is a function that maps variable indices to terms. Observe that
blanchet@33191
  1135
$\sigma$ is a function passed as argument and that Nitpick can't optimize it
blanchet@33191
  1136
away, because the recursive call for the \textit{Lam} case involves an altered
blanchet@33191
  1137
version. Also notice the \textit{lift} call, which increments the variable
blanchet@33191
  1138
indices when moving under a \textit{Lam}.
blanchet@33191
  1139
blanchet@33191
  1140
A reasonable property to expect of substitution is that it should leave closed
blanchet@33191
  1141
terms unchanged. Alas, even this simple property does not hold:
blanchet@33191
  1142
blanchet@33191
  1143
\pre
blanchet@33191
  1144
\textbf{lemma}~``$\lnot\,\textit{loose}~t~0 \,\Longrightarrow\, \textit{subst}~\sigma~t = t$'' \\
blanchet@33191
  1145
\textbf{nitpick} [\textit{verbose}] \\[2\smallskipamount]
blanchet@33191
  1146
\slshape
blanchet@38181
  1147
Trying 10 scopes: \nopagebreak \\
blanchet@33191
  1148
\hbox{}\qquad \textit{card~nat}~= 1, \textit{card tm}~= 1, and \textit{card} ``$\textit{nat} \Rightarrow \textit{tm}$'' = 1; \\
blanchet@33191
  1149
\hbox{}\qquad \textit{card~nat}~= 2, \textit{card tm}~= 2, and \textit{card} ``$\textit{nat} \Rightarrow \textit{tm}$'' = 2; \\
blanchet@33191
  1150
\hbox{}\qquad $\qquad\vdots$ \\[.5\smallskipamount]
blanchet@38181
  1151
\hbox{}\qquad \textit{card~nat}~= 10, \textit{card tm}~= 10, and \textit{card} ``$\textit{nat} \Rightarrow \textit{tm}$'' = 10. \\[2\smallskipamount]
blanchet@33191
  1152
Nitpick found a counterexample for \textit{card~nat}~= 6, \textit{card~tm}~= 6,
blanchet@33191
  1153
and \textit{card}~``$\textit{nat} \Rightarrow \textit{tm}$''~= 6: \\[2\smallskipamount]
blanchet@33191
  1154
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
  1155
\hbox{}\qquad\qquad $\sigma = \undef(\!\begin{aligned}[t]
blanchet@33191
  1156
& 0 := \textit{Var}~0,\>
blanchet@33191
  1157
  1 := \textit{Var}~0,\>
blanchet@33191
  1158
  2 := \textit{Var}~0, \\[-2pt]
blanchet@33191
  1159
& 3 := \textit{Var}~0,\>
blanchet@33191
  1160
  4 := \textit{Var}~0,\>
blanchet@33191
  1161
  5 := \textit{Var}~0)\end{aligned}$ \\
blanchet@33191
  1162
\hbox{}\qquad\qquad $t = \textit{Lam}~(\textit{Lam}~(\textit{Var}~1))$ \\[2\smallskipamount]
blanchet@38183
  1163
Total time: $3560$ ms.
blanchet@33191
  1164
\postw
blanchet@33191
  1165
blanchet@33191
  1166
Using \textit{eval}, we find out that $\textit{subst}~\sigma~t =
blanchet@33191
  1167
\textit{Lam}~(\textit{Lam}~(\textit{Var}~0))$. Using the traditional
blanchet@33191
  1168
$\lambda$-term notation, $t$~is
blanchet@33191
  1169
$\lambda x\, y.\> x$ whereas $\textit{subst}~\sigma~t$ is $\lambda x\, y.\> y$.
blanchet@35284
  1170
The bug is in \textit{subst\/}: The $\textit{lift}~(\sigma~m)~1$ call should be
blanchet@33191
  1171
replaced with $\textit{lift}~(\sigma~m)~0$.
blanchet@33191
  1172
blanchet@33191
  1173
An interesting aspect of Nitpick's verbose output is that it assigned inceasing
blanchet@38181
  1174
cardinalities from 1 to 10 to the type $\textit{nat} \Rightarrow \textit{tm}$.
blanchet@33191
  1175
For the formula of interest, knowing 6 values of that type was enough to find
blanchet@33191
  1176
the counterexample. Without boxing, $46\,656$ ($= 6^6$) values must be
blanchet@33191
  1177
considered, a hopeless undertaking:
blanchet@33191
  1178
blanchet@33191
  1179
\prew
blanchet@33191
  1180
\textbf{nitpick} [\textit{dont\_box}] \\[2\smallskipamount]
blanchet@38183
  1181
{\slshape Nitpick ran out of time after checking 3 of 10 scopes.}
blanchet@33191
  1182
\postw
blanchet@33191
  1183
blanchet@33191
  1184
{\looseness=-1
blanchet@33191
  1185
Boxing can be enabled or disabled globally or on a per-type basis using the
blanchet@35665
  1186
\textit{box} option. Nitpick usually performs reasonable choices about which
blanchet@35665
  1187
types should be boxed, but option tweaking sometimes helps. A related optimization,
blanchet@35665
  1188
``finalization,'' attempts to wrap functions that constant at all but finitely
blanchet@35665
  1189
many points (e.g., finite sets); see the documentation for the \textit{finalize}
blanchet@35665
  1190
option in \S\ref{scope-of-search} for details.
blanchet@33191
  1191
blanchet@33191
  1192
}
blanchet@33191
  1193
blanchet@33191
  1194
\subsection{Scope Monotonicity}
blanchet@33191
  1195
\label{scope-monotonicity}
blanchet@33191
  1196
blanchet@33191
  1197
The \textit{card} option (together with \textit{iter}, \textit{bisim\_depth},
blanchet@33191
  1198
and \textit{max}) controls which scopes are actually tested. In general, to
blanchet@33191
  1199
exhaust all models below a certain cardinality bound, the number of scopes that
blanchet@33191
  1200
Nitpick must consider increases exponentially with the number of type variables
blanchet@33191
  1201
(and \textbf{typedecl}'d types) occurring in the formula. Given the default
blanchet@38181
  1202
cardinality specification of 1--10, no fewer than $10^4 = 10\,000$ scopes must be
blanchet@33191
  1203
considered for a formula involving $'a$, $'b$, $'c$, and $'d$.
blanchet@33191
  1204
blanchet@33191
  1205
Fortunately, many formulas exhibit a property called \textsl{scope
blanchet@33191
  1206
monotonicity}, meaning that if the formula is falsifiable for a given scope,
blanchet@33191
  1207
it is also falsifiable for all larger scopes \cite[p.~165]{jackson-2006}.
blanchet@33191
  1208
blanchet@33191
  1209
Consider the formula
blanchet@33191
  1210
blanchet@33191
  1211
\prew
blanchet@33191
  1212
\textbf{lemma}~``$\textit{length~xs} = \textit{length~ys} \,\Longrightarrow\, \textit{rev}~(\textit{zip~xs~ys}) = \textit{zip~xs}~(\textit{rev~ys})$''
blanchet@33191
  1213
\postw
blanchet@33191
  1214
blanchet@33191
  1215
where \textit{xs} is of type $'a~\textit{list}$ and \textit{ys} is of type
blanchet@38181
  1216
$'b~\textit{list}$. A priori, Nitpick would need to consider $1\,000$ scopes to
blanchet@38274
  1217
exhaust the specification \textit{card}~= 1--10 (10 cardinalies for $'a$
blanchet@38274
  1218
$\times$ 10 cardinalities for $'b$ $\times$ 10 cardinalities for the datatypes).
blanchet@38274
  1219
However, our intuition tells us that any counterexample found with a small scope
blanchet@38274
  1220
would still be a counterexample in a larger scope---by simply ignoring the fresh
blanchet@38274
  1221
$'a$ and $'b$ values provided by the larger scope. Nitpick comes to the same
blanchet@38274
  1222
conclusion after a careful inspection of the formula and the relevant
blanchet@38274
  1223
definitions:
blanchet@33191
  1224
blanchet@33191
  1225
\prew
blanchet@33191
  1226
\textbf{nitpick}~[\textit{verbose}] \\[2\smallskipamount]
blanchet@33191
  1227
\slshape
blanchet@33191
  1228
The types ``\kern1pt$'a$'' and ``\kern1pt$'b$'' passed the monotonicity test.
blanchet@33191
  1229
Nitpick might be able to skip some scopes.
blanchet@33191
  1230
 \\[2\smallskipamount]
blanchet@38181
  1231
Trying 10 scopes: \\
blanchet@33191
  1232
\hbox{}\qquad \textit{card} $'a$~= 1, \textit{card} $'b$~= 1,
blanchet@33191
  1233
\textit{card} \textit{nat}~= 1, \textit{card} ``$('a \times {'}b)$
blanchet@35712
  1234
\textit{list\/}''~= 1, \\
blanchet@35712
  1235
\hbox{}\qquad\quad \textit{card} ``\kern1pt$'a$ \textit{list\/}''~= 1, and
blanchet@35712
  1236
\textit{card} ``\kern1pt$'b$ \textit{list\/}''~= 1. \\
blanchet@33191
  1237
\hbox{}\qquad \textit{card} $'a$~= 2, \textit{card} $'b$~= 2,
blanchet@33191
  1238
\textit{card} \textit{nat}~= 2, \textit{card} ``$('a \times {'}b)$
blanchet@35712
  1239
\textit{list\/}''~= 2, \\
blanchet@35712
  1240
\hbox{}\qquad\quad \textit{card} ``\kern1pt$'a$ \textit{list\/}''~= 2, and
blanchet@35712
  1241
\textit{card} ``\kern1pt$'b$ \textit{list\/}''~= 2. \\
blanchet@33191
  1242
\hbox{}\qquad $\qquad\vdots$ \\[.5\smallskipamount]
blanchet@38181
  1243
\hbox{}\qquad \textit{card} $'a$~= 10, \textit{card} $'b$~= 10,
blanchet@38181
  1244
\textit{card} \textit{nat}~= 10, \textit{card} ``$('a \times {'}b)$
blanchet@38181
  1245
\textit{list\/}''~= 10, \\
blanchet@38181
  1246
\hbox{}\qquad\quad \textit{card} ``\kern1pt$'a$ \textit{list\/}''~= 10, and
blanchet@38181
  1247
\textit{card} ``\kern1pt$'b$ \textit{list\/}''~= 10.
blanchet@33191
  1248
\\[2\smallskipamount]
blanchet@33191
  1249
Nitpick found a counterexample for
blanchet@33191
  1250
\textit{card} $'a$~= 5, \textit{card} $'b$~= 5,
blanchet@33191
  1251
\textit{card} \textit{nat}~= 5, \textit{card} ``$('a \times {'}b)$
blanchet@35712
  1252
\textit{list\/}''~= 5, \textit{card} ``\kern1pt$'a$ \textit{list\/}''~= 5, and
blanchet@35712
  1253
\textit{card} ``\kern1pt$'b$ \textit{list\/}''~= 5:
blanchet@33191
  1254
\\[2\smallskipamount]
blanchet@33191
  1255
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
  1256
\hbox{}\qquad\qquad $\textit{xs} = [a_1, a_2]$ \\
blanchet@35078
  1257
\hbox{}\qquad\qquad $\textit{ys} = [b_1, b_1]$ \\[2\smallskipamount]
blanchet@33191
  1258
Total time: 1636 ms.
blanchet@33191
  1259
\postw
blanchet@33191
  1260
blanchet@33191
  1261
In theory, it should be sufficient to test a single scope:
blanchet@33191
  1262
blanchet@33191
  1263
\prew
blanchet@38181
  1264
\textbf{nitpick}~[\textit{card}~= 10]
blanchet@33191
  1265
\postw
blanchet@33191
  1266
blanchet@33191
  1267
However, this is often less efficient in practice and may lead to overly complex
blanchet@33191
  1268
counterexamples.
blanchet@33191
  1269
blanchet@33191
  1270
If the monotonicity check fails but we believe that the formula is monotonic (or
blanchet@33191
  1271
we don't mind missing some counterexamples), we can pass the
blanchet@33191
  1272
\textit{mono} option. To convince yourself that this option is risky,
blanchet@33191
  1273
simply consider this example from \S\ref{skolemization}:
blanchet@33191
  1274
blanchet@33191
  1275
\prew
blanchet@33191
  1276
\textbf{lemma} ``$\exists g.\; \forall x\Colon 'b.~g~(f~x) = x
blanchet@33191
  1277
 \,\Longrightarrow\, \forall y\Colon {'}a.\; \exists x.~y = f~x$'' \\
blanchet@33191
  1278
\textbf{nitpick} [\textit{mono}] \\[2\smallskipamount]
blanchet@33191
  1279
{\slshape Nitpick found no counterexample.} \\[2\smallskipamount]
blanchet@33191
  1280
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1281
\slshape
blanchet@33191
  1282
Nitpick found a counterexample for \textit{card} $'a$~= 2 and \textit{card} $'b$~=~1: \\
blanchet@33191
  1283
\hbox{}\qquad $\vdots$
blanchet@33191
  1284
\postw
blanchet@33191
  1285
blanchet@33191
  1286
(It turns out the formula holds if and only if $\textit{card}~'a \le
blanchet@33191
  1287
\textit{card}~'b$.) Although this is rarely advisable, the automatic
blanchet@33191
  1288
monotonicity checks can be disabled by passing \textit{non\_mono}
blanchet@33191
  1289
(\S\ref{optimizations}).
blanchet@33191
  1290
blanchet@33191
  1291
As insinuated in \S\ref{natural-numbers-and-integers} and
blanchet@33191
  1292
\S\ref{inductive-datatypes}, \textit{nat}, \textit{int}, and inductive datatypes
blanchet@33191
  1293
are normally monotonic and treated as such. The same is true for record types,
blanchet@38274
  1294
\textit{rat}, and \textit{real}. Thus, given the
blanchet@38181
  1295
cardinality specification 1--10, a formula involving \textit{nat}, \textit{int},
blanchet@33191
  1296
\textit{int~list}, \textit{rat}, and \textit{rat~list} will lead Nitpick to
blanchet@38274
  1297
consider only 10~scopes instead of $10\,000$. On the other hand,
blanchet@38274
  1298
\textbf{typedef}s and quotient types are generally nonmonotonic.
blanchet@33191
  1299
blanchet@34982
  1300
\subsection{Inductive Properties}
blanchet@34982
  1301
\label{inductive-properties}
blanchet@34982
  1302
blanchet@34982
  1303
Inductive properties are a particular pain to prove, because the failure to
blanchet@34982
  1304
establish an induction step can mean several things:
blanchet@34982
  1305
%
blanchet@34982
  1306
\begin{enumerate}
blanchet@34982
  1307
\item The property is invalid.
blanchet@34982
  1308
\item The property is valid but is too weak to support the induction step.
blanchet@34982
  1309
\item The property is valid and strong enough; it's just that we haven't found
blanchet@34982
  1310
the proof yet.
blanchet@34982
  1311
\end{enumerate}
blanchet@34982
  1312
%
blanchet@34982
  1313
Depending on which scenario applies, we would take the appropriate course of
blanchet@34982
  1314
action:
blanchet@34982
  1315
%
blanchet@34982
  1316
\begin{enumerate}
blanchet@34982
  1317
\item Repair the statement of the property so that it becomes valid.
blanchet@34982
  1318
\item Generalize the property and/or prove auxiliary properties.
blanchet@34982
  1319
\item Work harder on a proof.
blanchet@34982
  1320
\end{enumerate}
blanchet@34982
  1321
%
blanchet@34982
  1322
How can we distinguish between the three scenarios? Nitpick's normal mode of
blanchet@34982
  1323
operation can often detect scenario 1, and Isabelle's automatic tactics help with
blanchet@34982
  1324
scenario 3. Using appropriate techniques, it is also often possible to use
blanchet@34982
  1325
Nitpick to identify scenario 2. Consider the following transition system,
blanchet@34982
  1326
in which natural numbers represent states:
blanchet@34982
  1327
blanchet@34982
  1328
\prew
blanchet@34982
  1329
\textbf{inductive\_set}~\textit{reach}~\textbf{where} \\
blanchet@34982
  1330
``$(4\Colon\textit{nat}) \in \textit{reach\/}$'' $\mid$ \\
blanchet@34982
  1331
``$\lbrakk n < 4;\> n \in \textit{reach\/}\rbrakk \,\Longrightarrow\, 3 * n + 1 \in \textit{reach\/}$'' $\mid$ \\
blanchet@34982
  1332
``$n \in \textit{reach} \,\Longrightarrow n + 2 \in \textit{reach\/}$''
blanchet@34982
  1333
\postw
blanchet@34982
  1334
blanchet@34982
  1335
We will try to prove that only even numbers are reachable:
blanchet@34982
  1336
blanchet@34982
  1337
\prew
blanchet@34982
  1338
\textbf{lemma}~``$n \in \textit{reach} \,\Longrightarrow\, 2~\textrm{dvd}~n$''
blanchet@34982
  1339
\postw
blanchet@34982
  1340
blanchet@34982
  1341
Does this property hold? Nitpick cannot find a counterexample within 30 seconds,
blanchet@34982
  1342
so let's attempt a proof by induction:
blanchet@34982
  1343
blanchet@34982
  1344
\prew
blanchet@34982
  1345
\textbf{apply}~(\textit{induct~set}{:}~\textit{reach\/}) \\
blanchet@34982
  1346
\textbf{apply}~\textit{auto}
blanchet@34982
  1347
\postw
blanchet@34982
  1348
blanchet@34982
  1349
This leaves us in the following proof state:
blanchet@34982
  1350
blanchet@34982
  1351
\prew
blanchet@34982
  1352
{\slshape goal (2 subgoals): \\
blanchet@34982
  1353
\phantom{0}1. ${\bigwedge}n.\;\, \lbrakk n \in \textit{reach\/};\, n < 4;\, 2~\textsl{dvd}~n\rbrakk \,\Longrightarrow\, 2~\textsl{dvd}~\textit{Suc}~(3 * n)$ \\
blanchet@34982
  1354
\phantom{0}2. ${\bigwedge}n.\;\, \lbrakk n \in \textit{reach\/};\, 2~\textsl{dvd}~n\rbrakk \,\Longrightarrow\, 2~\textsl{dvd}~\textit{Suc}~(\textit{Suc}~n)$
blanchet@34982
  1355
}
blanchet@34982
  1356
\postw
blanchet@34982
  1357
blanchet@34982
  1358
If we run Nitpick on the first subgoal, it still won't find any
blanchet@34982
  1359
counterexample; and yet, \textit{auto} fails to go further, and \textit{arith}
blanchet@34982
  1360
is helpless. However, notice the $n \in \textit{reach}$ assumption, which
blanchet@34982
  1361
strengthens the induction hypothesis but is not immediately usable in the proof.
blanchet@34982
  1362
If we remove it and invoke Nitpick, this time we get a counterexample:
blanchet@34982
  1363
blanchet@34982
  1364
\prew
blanchet@34982
  1365
\textbf{apply}~(\textit{thin\_tac}~``$n \in \textit{reach\/}$'') \\
blanchet@34982
  1366
\textbf{nitpick} \\[2\smallskipamount]
blanchet@34982
  1367
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@34982
  1368
\hbox{}\qquad Skolem constant: \nopagebreak \\
blanchet@34982
  1369
\hbox{}\qquad\qquad $n = 0$
blanchet@34982
  1370
\postw
blanchet@34982
  1371
blanchet@34982
  1372
Indeed, 0 < 4, 2 divides 0, but 2 does not divide 1. We can use this information
blanchet@34982
  1373
to strength the lemma:
blanchet@34982
  1374
blanchet@34982
  1375
\prew
blanchet@34982
  1376
\textbf{lemma}~``$n \in \textit{reach} \,\Longrightarrow\, 2~\textrm{dvd}~n \mathrel{\lor} n \not= 0$''
blanchet@34982
  1377
\postw
blanchet@34982
  1378
blanchet@34982
  1379
Unfortunately, the proof by induction still gets stuck, except that Nitpick now
blanchet@34982
  1380
finds the counterexample $n = 2$. We generalize the lemma further to
blanchet@34982
  1381
blanchet@34982
  1382
\prew
blanchet@34982
  1383
\textbf{lemma}~``$n \in \textit{reach} \,\Longrightarrow\, 2~\textrm{dvd}~n \mathrel{\lor} n \ge 4$''
blanchet@34982
  1384
\postw
blanchet@34982
  1385
blanchet@34982
  1386
and this time \textit{arith} can finish off the subgoals.
blanchet@34982
  1387
blanchet@34982
  1388
A similar technique can be employed for structural induction. The
blanchet@35180
  1389
following mini formalization of full binary trees will serve as illustration:
blanchet@34982
  1390
blanchet@34982
  1391
\prew
blanchet@34982
  1392
\textbf{datatype} $\kern1pt'a$~\textit{bin\_tree} = $\textit{Leaf}~{\kern1pt'a}$ $\mid$ $\textit{Branch}$ ``\kern1pt$'a$ \textit{bin\_tree}'' ``\kern1pt$'a$ \textit{bin\_tree}'' \\[2\smallskipamount]
blanchet@34982
  1393
\textbf{primrec}~\textit{labels}~\textbf{where} \\
blanchet@34982
  1394
``$\textit{labels}~(\textit{Leaf}~a) = \{a\}$'' $\mid$ \\
blanchet@34982
  1395
``$\textit{labels}~(\textit{Branch}~t~u) = \textit{labels}~t \mathrel{\cup} \textit{labels}~u$'' \\[2\smallskipamount]
blanchet@34982
  1396
\textbf{primrec}~\textit{swap}~\textbf{where} \\
blanchet@34982
  1397
``$\textit{swap}~(\textit{Leaf}~c)~a~b =$ \\
blanchet@34982
  1398
\phantom{``}$(\textrm{if}~c = a~\textrm{then}~\textit{Leaf}~b~\textrm{else~if}~c = b~\textrm{then}~\textit{Leaf}~a~\textrm{else}~\textit{Leaf}~c)$'' $\mid$ \\
blanchet@34982
  1399
``$\textit{swap}~(\textit{Branch}~t~u)~a~b = \textit{Branch}~(\textit{swap}~t~a~b)~(\textit{swap}~u~a~b)$''
blanchet@34982
  1400
\postw
blanchet@34982
  1401
blanchet@34982
  1402
The \textit{labels} function returns the set of labels occurring on leaves of a
blanchet@34982
  1403
tree, and \textit{swap} exchanges two labels. Intuitively, if two distinct
blanchet@34982
  1404
labels $a$ and $b$ occur in a tree $t$, they should also occur in the tree
blanchet@34982
  1405
obtained by swapping $a$ and $b$:
blanchet@34982
  1406
blanchet@34982
  1407
\prew
blanchet@35180
  1408
\textbf{lemma} $``\{a, b\} \subseteq \textit{labels}~t \,\Longrightarrow\, \textit{labels}~(\textit{swap}~t~a~b) = \textit{labels}~t$''
blanchet@34982
  1409
\postw
blanchet@34982
  1410
blanchet@34982
  1411
Nitpick can't find any counterexample, so we proceed with induction
blanchet@34982
  1412
(this time favoring a more structured style):
blanchet@34982
  1413
blanchet@34982
  1414
\prew
blanchet@34982
  1415
\textbf{proof}~(\textit{induct}~$t$) \\
blanchet@34982
  1416
\hbox{}\quad \textbf{case}~\textit{Leaf}~\textbf{thus}~\textit{?case}~\textbf{by}~\textit{simp} \\
blanchet@34982
  1417
\textbf{next} \\
blanchet@34982
  1418
\hbox{}\quad \textbf{case}~$(\textit{Branch}~t~u)$~\textbf{thus} \textit{?case}
blanchet@34982
  1419
\postw
blanchet@34982
  1420
blanchet@34982
  1421
Nitpick can't find any counterexample at this point either, but it makes the
blanchet@34982
  1422
following suggestion:
blanchet@34982
  1423
blanchet@34982
  1424
\prew
blanchet@34982
  1425
\slshape
blanchet@35178
  1426
Hint: To check that the induction hypothesis is general enough, try this command:
blanchet@35183
  1427
\textbf{nitpick}~[\textit{non\_std}, \textit{show\_all}].
blanchet@34982
  1428
\postw
blanchet@34982
  1429
blanchet@34982
  1430
If we follow the hint, we get a ``nonstandard'' counterexample for the step:
blanchet@34982
  1431
blanchet@34982
  1432
\prew
blanchet@35180
  1433
\slshape Nitpick found a nonstandard counterexample for \textit{card} $'a$ = 3: \\[2\smallskipamount]
blanchet@34982
  1434
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
  1435
\hbox{}\qquad\qquad $a = a_1$ \\
blanchet@35078
  1436
\hbox{}\qquad\qquad $b = a_2$ \\
blanchet@35078
  1437
\hbox{}\qquad\qquad $t = \xi_1$ \\
blanchet@35078
  1438
\hbox{}\qquad\qquad $u = \xi_2$ \\
blanchet@35180
  1439
\hbox{}\qquad Datatype: \nopagebreak \\
blanchet@35180
  1440
\hbox{}\qquad\qquad $\alpha~\textit{btree} = \{\xi_1 \mathbin{=} \textit{Branch}~\xi_1~\xi_1,\> \xi_2 \mathbin{=} \textit{Branch}~\xi_2~\xi_2,\> \textit{Branch}~\xi_1~\xi_2\}$ \\
blanchet@34982
  1441
\hbox{}\qquad {\slshape Constants:} \nopagebreak \\
blanchet@34982
  1442
\hbox{}\qquad\qquad $\textit{labels} = \undef
blanchet@34982
  1443
    (\!\begin{aligned}[t]%
blanchet@35180
  1444
    & \xi_1 := \{a_2, a_3\},\> \xi_2 := \{a_1\},\> \\[-2pt]
blanchet@35180
  1445
    & \textit{Branch}~\xi_1~\xi_2 := \{a_1, a_2, a_3\})\end{aligned}$ \\
blanchet@34982
  1446
\hbox{}\qquad\qquad $\lambda x_1.\> \textit{swap}~x_1~a~b = \undef
blanchet@34982
  1447
    (\!\begin{aligned}[t]%
blanchet@35078
  1448
    & \xi_1 := \xi_2,\> \xi_2 := \xi_2, \\[-2pt]
blanchet@35180
  1449
    & \textit{Branch}~\xi_1~\xi_2 := \xi_2)\end{aligned}$ \\[2\smallskipamount]
blanchet@36126
  1450
The existence of a nonstandard model suggests that the induction hypothesis is not general enough or may even
blanchet@36126
  1451
be wrong. See the Nitpick manual's ``Inductive Properties'' section for details (``\textit{isabelle doc nitpick}'').
blanchet@34982
  1452
\postw
blanchet@34982
  1453
blanchet@34982
  1454
Reading the Nitpick manual is a most excellent idea.
blanchet@35183
  1455
But what's going on? The \textit{non\_std} option told the tool to look for
blanchet@35183
  1456
nonstandard models of binary trees, which means that new ``nonstandard'' trees
blanchet@35183
  1457
$\xi_1, \xi_2, \ldots$, are now allowed in addition to the standard trees
blanchet@35183
  1458
generated by the \textit{Leaf} and \textit{Branch} constructors.%
blanchet@34982
  1459
\footnote{Notice the similarity between allowing nonstandard trees here and
blanchet@34982
  1460
allowing unreachable states in the preceding example (by removing the ``$n \in
blanchet@34982
  1461
\textit{reach\/}$'' assumption). In both cases, we effectively enlarge the
blanchet@34982
  1462
set of objects over which the induction is performed while doing the step
blanchet@35078
  1463
in order to test the induction hypothesis's strength.}
blanchet@35180
  1464
Unlike standard trees, these new trees contain cycles. We will see later that
blanchet@35180
  1465
every property of acyclic trees that can be proved without using induction also
blanchet@35180
  1466
holds for cyclic trees. Hence,
blanchet@34982
  1467
%
blanchet@34982
  1468
\begin{quote}
blanchet@34982
  1469
\textsl{If the induction
blanchet@34982
  1470
hypothesis is strong enough, the induction step will hold even for nonstandard
blanchet@34982
  1471
objects, and Nitpick won't find any nonstandard counterexample.}
blanchet@34982
  1472
\end{quote}
blanchet@34982
  1473
%
blanchet@35180
  1474
But here the tool find some nonstandard trees $t = \xi_1$
blanchet@35180
  1475
and $u = \xi_2$ such that $a \notin \textit{labels}~t$, $b \in
blanchet@35180
  1476
\textit{labels}~t$, $a \in \textit{labels}~u$, and $b \notin \textit{labels}~u$.
blanchet@34982
  1477
Because neither tree contains both $a$ and $b$, the induction hypothesis tells
blanchet@34982
  1478
us nothing about the labels of $\textit{swap}~t~a~b$ and $\textit{swap}~u~a~b$,
blanchet@34982
  1479
and as a result we know nothing about the labels of the tree
blanchet@34982
  1480
$\textit{swap}~(\textit{Branch}~t~u)~a~b$, which by definition equals
blanchet@34982
  1481
$\textit{Branch}$ $(\textit{swap}~t~a~b)$ $(\textit{swap}~u~a~b)$, whose
blanchet@34982
  1482
labels are $\textit{labels}$ $(\textit{swap}~t~a~b) \mathrel{\cup}
blanchet@34982
  1483
\textit{labels}$ $(\textit{swap}~u~a~b)$.
blanchet@34982
  1484
blanchet@34982
  1485
The solution is to ensure that we always know what the labels of the subtrees
blanchet@34982
  1486
are in the inductive step, by covering the cases where $a$ and/or~$b$ is not in
blanchet@34982
  1487
$t$ in the statement of the lemma:
blanchet@34982
  1488
blanchet@34982
  1489
\prew
blanchet@34982
  1490
\textbf{lemma} ``$\textit{labels}~(\textit{swap}~t~a~b) = {}$ \\
blanchet@34982
  1491
\phantom{\textbf{lemma} ``}$(\textrm{if}~a \in \textit{labels}~t~\textrm{then}$ \nopagebreak \\
blanchet@34982
  1492
\phantom{\textbf{lemma} ``(\quad}$\textrm{if}~b \in \textit{labels}~t~\textrm{then}~\textit{labels}~t~\textrm{else}~(\textit{labels}~t - \{a\}) \mathrel{\cup} \{b\}$ \\
blanchet@34982
  1493
\phantom{\textbf{lemma} ``(}$\textrm{else}$ \\
blanchet@34982
  1494
\phantom{\textbf{lemma} ``(\quad}$\textrm{if}~b \in \textit{labels}~t~\textrm{then}~(\textit{labels}~t - \{b\}) \mathrel{\cup} \{a\}~\textrm{else}~\textit{labels}~t)$''
blanchet@34982
  1495
\postw
blanchet@34982
  1496
blanchet@34982
  1497
This time, Nitpick won't find any nonstandard counterexample, and we can perform
blanchet@35078
  1498
the induction step using \textit{auto}.
blanchet@34982
  1499
blanchet@33191
  1500
\section{Case Studies}
blanchet@33191
  1501
\label{case-studies}
blanchet@33191
  1502
blanchet@33191
  1503
As a didactic device, the previous section focused mostly on toy formulas whose
blanchet@33191
  1504
validity can easily be assessed just by looking at the formula. We will now
blanchet@33191
  1505
review two somewhat more realistic case studies that are within Nitpick's
blanchet@33191
  1506
reach:\ a context-free grammar modeled by mutually inductive sets and a
blanchet@33191
  1507
functional implementation of AA trees. The results presented in this
blanchet@33191
  1508
section were produced with the following settings:
blanchet@33191
  1509
blanchet@33191
  1510
\prew
blanchet@36268
  1511
\textbf{nitpick\_params} [\textit{max\_potential}~= 0]
blanchet@33191
  1512
\postw
blanchet@33191
  1513
blanchet@33191
  1514
\subsection{A Context-Free Grammar}
blanchet@33191
  1515
\label{a-context-free-grammar}
blanchet@33191
  1516
blanchet@33191
  1517
Our first case study is taken from section 7.4 in the Isabelle tutorial
blanchet@33191
  1518
\cite{isa-tutorial}. The following grammar, originally due to Hopcroft and
blanchet@33191
  1519
Ullman, produces all strings with an equal number of $a$'s and $b$'s:
blanchet@33191
  1520
blanchet@33191
  1521
\prew
blanchet@33191
  1522
\begin{tabular}{@{}r@{$\;\,$}c@{$\;\,$}l@{}}
blanchet@33191
  1523
$S$ & $::=$ & $\epsilon \mid bA \mid aB$ \\
blanchet@33191
  1524
$A$ & $::=$ & $aS \mid bAA$ \\
blanchet@33191
  1525
$B$ & $::=$ & $bS \mid aBB$
blanchet@33191
  1526
\end{tabular}
blanchet@33191
  1527
\postw
blanchet@33191
  1528
blanchet@33191
  1529
The intuition behind the grammar is that $A$ generates all string with one more
blanchet@33191
  1530
$a$ than $b$'s and $B$ generates all strings with one more $b$ than $a$'s.
blanchet@33191
  1531
blanchet@33191
  1532
The alphabet consists exclusively of $a$'s and $b$'s:
blanchet@33191
  1533
blanchet@33191
  1534
\prew
blanchet@33191
  1535
\textbf{datatype} \textit{alphabet}~= $a$ $\mid$ $b$
blanchet@33191
  1536
\postw
blanchet@33191
  1537
blanchet@33191
  1538
Strings over the alphabet are represented by \textit{alphabet list}s.
blanchet@33191
  1539
Nonterminals in the grammar become sets of strings. The production rules
blanchet@33191
  1540
presented above can be expressed as a mutually inductive definition:
blanchet@33191
  1541
blanchet@33191
  1542
\prew
blanchet@33191
  1543
\textbf{inductive\_set} $S$ \textbf{and} $A$ \textbf{and} $B$ \textbf{where} \\
blanchet@33191
  1544
\textit{R1}:\kern.4em ``$[] \in S$'' $\,\mid$ \\
blanchet@33191
  1545
\textit{R2}:\kern.4em ``$w \in A\,\Longrightarrow\, b \mathbin{\#} w \in S$'' $\,\mid$ \\
blanchet@33191
  1546
\textit{R3}:\kern.4em ``$w \in B\,\Longrightarrow\, a \mathbin{\#} w \in S$'' $\,\mid$ \\
blanchet@33191
  1547
\textit{R4}:\kern.4em ``$w \in S\,\Longrightarrow\, a \mathbin{\#} w \in A$'' $\,\mid$ \\
blanchet@33191
  1548
\textit{R5}:\kern.4em ``$w \in S\,\Longrightarrow\, b \mathbin{\#} w \in S$'' $\,\mid$ \\
blanchet@33191
  1549
\textit{R6}:\kern.4em ``$\lbrakk v \in B;\> v \in B\rbrakk \,\Longrightarrow\, a \mathbin{\#} v \mathbin{@} w \in B$''
blanchet@33191
  1550
\postw
blanchet@33191
  1551
blanchet@33191
  1552
The conversion of the grammar into the inductive definition was done manually by
blanchet@33191
  1553
Joe Blow, an underpaid undergraduate student. As a result, some errors might
blanchet@33191
  1554
have sneaked in.
blanchet@33191
  1555
blanchet@33191
  1556
Debugging faulty specifications is at the heart of Nitpick's \textsl{raison
blanchet@33191
  1557
d'\^etre}. A good approach is to state desirable properties of the specification
blanchet@33191
  1558
(here, that $S$ is exactly the set of strings over $\{a, b\}$ with as many $a$'s
blanchet@33191
  1559
as $b$'s) and check them with Nitpick. If the properties are correctly stated,
blanchet@33191
  1560
counterexamples will point to bugs in the specification. For our grammar
blanchet@33191
  1561
example, we will proceed in two steps, separating the soundness and the
blanchet@33191
  1562
completeness of the set $S$. First, soundness:
blanchet@33191
  1563
blanchet@33191
  1564
\prew
blanchet@35284
  1565
\textbf{theorem}~\textit{S\_sound\/}: \\
blanchet@33191
  1566
``$w \in S \longrightarrow \textit{length}~[x\mathbin{\leftarrow} w.\; x = a] =
blanchet@33191
  1567
  \textit{length}~[x\mathbin{\leftarrow} w.\; x = b]$'' \\
blanchet@33191
  1568
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1569
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
  1570
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
  1571
\hbox{}\qquad\qquad $w = [b]$
blanchet@33191
  1572
\postw
blanchet@33191
  1573
blanchet@33191
  1574
It would seem that $[b] \in S$. How could this be? An inspection of the
blanchet@33191
  1575
introduction rules reveals that the only rule with a right-hand side of the form
blanchet@33191
  1576
$b \mathbin{\#} {\ldots} \in S$ that could have introduced $[b]$ into $S$ is
blanchet@33191
  1577
\textit{R5}:
blanchet@33191
  1578
blanchet@33191
  1579
\prew
blanchet@33191
  1580
``$w \in S\,\Longrightarrow\, b \mathbin{\#} w \in S$''
blanchet@33191
  1581
\postw
blanchet@33191
  1582
blanchet@33191
  1583
On closer inspection, we can see that this rule is wrong. To match the
blanchet@33191
  1584
production $B ::= bS$, the second $S$ should be a $B$. We fix the typo and try
blanchet@33191
  1585
again:
blanchet@33191
  1586
blanchet@33191
  1587
\prew
blanchet@33191
  1588
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1589
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
  1590
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
  1591
\hbox{}\qquad\qquad $w = [a, a, b]$
blanchet@33191
  1592
\postw
blanchet@33191
  1593
blanchet@33191
  1594
Some detective work is necessary to find out what went wrong here. To get $[a,
blanchet@33191
  1595
a, b] \in S$, we need $[a, b] \in B$ by \textit{R3}, which in turn can only come
blanchet@33191
  1596
from \textit{R6}:
blanchet@33191
  1597
blanchet@33191
  1598
\prew
blanchet@33191
  1599
``$\lbrakk v \in B;\> v \in B\rbrakk \,\Longrightarrow\, a \mathbin{\#} v \mathbin{@} w \in B$''
blanchet@33191
  1600
\postw
blanchet@33191
  1601
blanchet@33191
  1602
Now, this formula must be wrong: The same assumption occurs twice, and the
blanchet@33191
  1603
variable $w$ is unconstrained. Clearly, one of the two occurrences of $v$ in
blanchet@33191
  1604
the assumptions should have been a $w$.
blanchet@33191
  1605
blanchet@33191
  1606
With the correction made, we don't get any counterexample from Nitpick. Let's
blanchet@33191
  1607
move on and check completeness:
blanchet@33191
  1608
blanchet@33191
  1609
\prew
blanchet@33191
  1610
\textbf{theorem}~\textit{S\_complete}: \\
blanchet@33191
  1611
``$\textit{length}~[x\mathbin{\leftarrow} w.\; x = a] =
blanchet@33191
  1612
   \textit{length}~[x\mathbin{\leftarrow} w.\; x = b]
blanchet@33191
  1613
  \longrightarrow w \in S$'' \\
blanchet@33191
  1614
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1615
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
  1616
\hbox{}\qquad Free variable: \nopagebreak \\
blanchet@33191
  1617
\hbox{}\qquad\qquad $w = [b, b, a, a]$
blanchet@33191
  1618
\postw
blanchet@33191
  1619
blanchet@33191
  1620
Apparently, $[b, b, a, a] \notin S$, even though it has the same numbers of
blanchet@33191
  1621
$a$'s and $b$'s. But since our inductive definition passed the soundness check,
blanchet@33191
  1622
the introduction rules we have are probably correct. Perhaps we simply lack an
blanchet@33191
  1623
introduction rule. Comparing the grammar with the inductive definition, our
blanchet@33191
  1624
suspicion is confirmed: Joe Blow simply forgot the production $A ::= bAA$,
blanchet@33191
  1625
without which the grammar cannot generate two or more $b$'s in a row. So we add
blanchet@33191
  1626
the rule
blanchet@33191
  1627
blanchet@33191
  1628
\prew
blanchet@33191
  1629
``$\lbrakk v \in A;\> w \in A\rbrakk \,\Longrightarrow\, b \mathbin{\#} v \mathbin{@} w \in A$''
blanchet@33191
  1630
\postw
blanchet@33191
  1631
blanchet@33191
  1632
With this last change, we don't get any counterexamples from Nitpick for either
blanchet@33191
  1633
soundness or completeness. We can even generalize our result to cover $A$ and
blanchet@33191
  1634
$B$ as well:
blanchet@33191
  1635
blanchet@33191
  1636
\prew
blanchet@33191
  1637
\textbf{theorem} \textit{S\_A\_B\_sound\_and\_complete}: \\
blanchet@33191
  1638
``$w \in S \longleftrightarrow \textit{length}~[x \mathbin{\leftarrow} w.\; x = a] = \textit{length}~[x \mathbin{\leftarrow} w.\; x = b]$'' \\
blanchet@33191
  1639
``$w \in A \longleftrightarrow \textit{length}~[x \mathbin{\leftarrow} w.\; x = a] = \textit{length}~[x \mathbin{\leftarrow} w.\; x = b] + 1$'' \\
blanchet@33191
  1640
``$w \in B \longleftrightarrow \textit{length}~[x \mathbin{\leftarrow} w.\; x = b] = \textit{length}~[x \mathbin{\leftarrow} w.\; x = a] + 1$'' \\
blanchet@33191
  1641
\textbf{nitpick} \\[2\smallskipamount]
blanchet@38183
  1642
\slshape Nitpick found no counterexample.
blanchet@33191
  1643
\postw
blanchet@33191
  1644
blanchet@33191
  1645
\subsection{AA Trees}
blanchet@33191
  1646
\label{aa-trees}
blanchet@33191
  1647
blanchet@33191
  1648
AA trees are a kind of balanced trees discovered by Arne Andersson that provide
blanchet@33191
  1649
similar performance to red-black trees, but with a simpler implementation
blanchet@33191
  1650
\cite{andersson-1993}. They can be used to store sets of elements equipped with
blanchet@33191
  1651
a total order $<$. We start by defining the datatype and some basic extractor
blanchet@33191
  1652
functions:
blanchet@33191
  1653
blanchet@33191
  1654
\prew
blanchet@34982
  1655
\textbf{datatype} $'a$~\textit{aa\_tree} = \\
blanchet@34982
  1656
\hbox{}\quad $\Lambda$ $\mid$ $N$ ``\kern1pt$'a\Colon \textit{linorder}$'' \textit{nat} ``\kern1pt$'a$ \textit{aa\_tree}'' ``\kern1pt$'a$ \textit{aa\_tree}''  \\[2\smallskipamount]
blanchet@33191
  1657
\textbf{primrec} \textit{data} \textbf{where} \\
blanchet@33191
  1658
``$\textit{data}~\Lambda = \undef$'' $\,\mid$ \\
blanchet@33191
  1659
``$\textit{data}~(N~x~\_~\_~\_) = x$'' \\[2\smallskipamount]
blanchet@33191
  1660
\textbf{primrec} \textit{dataset} \textbf{where} \\
blanchet@33191
  1661
``$\textit{dataset}~\Lambda = \{\}$'' $\,\mid$ \\
blanchet@33191
  1662
``$\textit{dataset}~(N~x~\_~t~u) = \{x\} \cup \textit{dataset}~t \mathrel{\cup} \textit{dataset}~u$'' \\[2\smallskipamount]
blanchet@33191
  1663
\textbf{primrec} \textit{level} \textbf{where} \\
blanchet@33191
  1664
``$\textit{level}~\Lambda = 0$'' $\,\mid$ \\
blanchet@33191
  1665
``$\textit{level}~(N~\_~k~\_~\_) = k$'' \\[2\smallskipamount]
blanchet@33191
  1666
\textbf{primrec} \textit{left} \textbf{where} \\
blanchet@33191
  1667
``$\textit{left}~\Lambda = \Lambda$'' $\,\mid$ \\
blanchet@33191
  1668
``$\textit{left}~(N~\_~\_~t~\_) = t$'' \\[2\smallskipamount]
blanchet@33191
  1669
\textbf{primrec} \textit{right} \textbf{where} \\
blanchet@33191
  1670
``$\textit{right}~\Lambda = \Lambda$'' $\,\mid$ \\
blanchet@33191
  1671
``$\textit{right}~(N~\_~\_~\_~u) = u$''
blanchet@33191
  1672
\postw
blanchet@33191
  1673
blanchet@33191
  1674
The wellformedness criterion for AA trees is fairly complex. Wikipedia states it
blanchet@33191
  1675
as follows \cite{wikipedia-2009-aa-trees}:
blanchet@33191
  1676
blanchet@33191
  1677
\kern.2\parskip %% TYPESETTING
blanchet@33191
  1678
blanchet@33191
  1679
\pre
blanchet@33191
  1680
Each node has a level field, and the following invariants must remain true for
blanchet@33191
  1681
the tree to be valid:
blanchet@33191
  1682
blanchet@33191
  1683
\raggedright
blanchet@33191
  1684
blanchet@33191
  1685
\kern-.4\parskip %% TYPESETTING
blanchet@33191
  1686
blanchet@33191
  1687
\begin{enum}
blanchet@33191
  1688
\item[]
blanchet@33191
  1689
\begin{enum}
blanchet@33191
  1690
\item[1.] The level of a leaf node is one.
blanchet@33191
  1691
\item[2.] The level of a left child is strictly less than that of its parent.
blanchet@33191
  1692
\item[3.] The level of a right child is less than or equal to that of its parent.
blanchet@33191
  1693
\item[4.] The level of a right grandchild is strictly less than that of its grandparent.
blanchet@33191
  1694
\item[5.] Every node of level greater than one must have two children.
blanchet@33191
  1695
\end{enum}
blanchet@33191
  1696
\end{enum}
blanchet@33191
  1697
\post
blanchet@33191
  1698
blanchet@33191
  1699
\kern.4\parskip %% TYPESETTING
blanchet@33191
  1700
blanchet@33191
  1701
The \textit{wf} predicate formalizes this description:
blanchet@33191
  1702
blanchet@33191
  1703
\prew
blanchet@33191
  1704
\textbf{primrec} \textit{wf} \textbf{where} \\
blanchet@33191
  1705
``$\textit{wf}~\Lambda = \textit{True}$'' $\,\mid$ \\
blanchet@33191
  1706
``$\textit{wf}~(N~\_~k~t~u) =$ \\
blanchet@33191
  1707
\phantom{``}$(\textrm{if}~t = \Lambda~\textrm{then}$ \\
blanchet@33191
  1708
\phantom{``$(\quad$}$k = 1 \mathrel{\land} (u = \Lambda \mathrel{\lor} (\textit{level}~u = 1 \mathrel{\land} \textit{left}~u = \Lambda \mathrel{\land} \textit{right}~u = \Lambda))$ \\
blanchet@33191
  1709
\phantom{``$($}$\textrm{else}$ \\
blanchet@33193
  1710
\hbox{}\phantom{``$(\quad$}$\textit{wf}~t \mathrel{\land} \textit{wf}~u
blanchet@33191
  1711
\mathrel{\land} u \not= \Lambda \mathrel{\land} \textit{level}~t < k
blanchet@33193
  1712
\mathrel{\land} \textit{level}~u \le k$ \\
blanchet@33193
  1713
\hbox{}\phantom{``$(\quad$}${\land}\; \textit{level}~(\textit{right}~u) < k)$''
blanchet@33191
  1714
\postw
blanchet@33191
  1715
blanchet@33191
  1716
Rebalancing the tree upon insertion and removal of elements is performed by two
blanchet@33191
  1717
auxiliary functions called \textit{skew} and \textit{split}, defined below:
blanchet@33191
  1718
blanchet@33191
  1719
\prew
blanchet@33191
  1720
\textbf{primrec} \textit{skew} \textbf{where} \\
blanchet@33191
  1721
``$\textit{skew}~\Lambda = \Lambda$'' $\,\mid$ \\
blanchet@33191
  1722
``$\textit{skew}~(N~x~k~t~u) = {}$ \\
blanchet@33191
  1723
\phantom{``}$(\textrm{if}~t \not= \Lambda \mathrel{\land} k =
blanchet@33191
  1724
\textit{level}~t~\textrm{then}$ \\
blanchet@33191
  1725
\phantom{``(\quad}$N~(\textit{data}~t)~k~(\textit{left}~t)~(N~x~k~
blanchet@33191
  1726
(\textit{right}~t)~u)$ \\
blanchet@33191
  1727
\phantom{``(}$\textrm{else}$ \\
blanchet@33191
  1728
\phantom{``(\quad}$N~x~k~t~u)$''
blanchet@33191
  1729
\postw
blanchet@33191
  1730
blanchet@33191
  1731
\prew
blanchet@33191
  1732
\textbf{primrec} \textit{split} \textbf{where} \\
blanchet@33191
  1733
``$\textit{split}~\Lambda = \Lambda$'' $\,\mid$ \\
blanchet@33191
  1734
``$\textit{split}~(N~x~k~t~u) = {}$ \\
blanchet@33191
  1735
\phantom{``}$(\textrm{if}~u \not= \Lambda \mathrel{\land} k =
blanchet@33191
  1736
\textit{level}~(\textit{right}~u)~\textrm{then}$ \\
blanchet@33191
  1737
\phantom{``(\quad}$N~(\textit{data}~u)~(\textit{Suc}~k)~
blanchet@33191
  1738
(N~x~k~t~(\textit{left}~u))~(\textit{right}~u)$ \\
blanchet@33191
  1739
\phantom{``(}$\textrm{else}$ \\
blanchet@33191
  1740
\phantom{``(\quad}$N~x~k~t~u)$''
blanchet@33191
  1741
\postw
blanchet@33191
  1742
blanchet@33191
  1743
Performing a \textit{skew} or a \textit{split} should have no impact on the set
blanchet@33191
  1744
of elements stored in the tree:
blanchet@33191
  1745
blanchet@33191
  1746
\prew
blanchet@35284
  1747
\textbf{theorem}~\textit{dataset\_skew\_split\/}:\\
blanchet@33191
  1748
``$\textit{dataset}~(\textit{skew}~t) = \textit{dataset}~t$'' \\
blanchet@33191
  1749
``$\textit{dataset}~(\textit{split}~t) = \textit{dataset}~t$'' \\
blanchet@33191
  1750
\textbf{nitpick} \\[2\smallskipamount]
blanchet@38183
  1751
{\slshape Nitpick ran out of time after checking 9 of 10 scopes.}
blanchet@33191
  1752
\postw
blanchet@33191
  1753
blanchet@40147
  1754
Furthermore, applying \textit{skew} or \textit{split} on a well-formed tree
blanchet@33191
  1755
should not alter the tree:
blanchet@33191
  1756
blanchet@33191
  1757
\prew
blanchet@35284
  1758
\textbf{theorem}~\textit{wf\_skew\_split\/}:\\
blanchet@33191
  1759
``$\textit{wf}~t\,\Longrightarrow\, \textit{skew}~t = t$'' \\
blanchet@33191
  1760
``$\textit{wf}~t\,\Longrightarrow\, \textit{split}~t = t$'' \\
blanchet@33191
  1761
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1762
{\slshape Nitpick found no counterexample.}
blanchet@33191
  1763
\postw
blanchet@33191
  1764
blanchet@33191
  1765
Insertion is implemented recursively. It preserves the sort order:
blanchet@33191
  1766
blanchet@33191
  1767
\prew
blanchet@33191
  1768
\textbf{primrec}~\textit{insort} \textbf{where} \\
blanchet@33191
  1769
``$\textit{insort}~\Lambda~x = N~x~1~\Lambda~\Lambda$'' $\,\mid$ \\
blanchet@33191
  1770
``$\textit{insort}~(N~y~k~t~u)~x =$ \\
blanchet@33191
  1771
\phantom{``}$({*}~(\textit{split} \circ \textit{skew})~{*})~(N~y~k~(\textrm{if}~x < y~\textrm{then}~\textit{insort}~t~x~\textrm{else}~t)$ \\
blanchet@33191
  1772
\phantom{``$({*}~(\textit{split} \circ \textit{skew})~{*})~(N~y~k~$}$(\textrm{if}~x > y~\textrm{then}~\textit{insort}~u~x~\textrm{else}~u))$''
blanchet@33191
  1773
\postw
blanchet@33191
  1774
blanchet@33191
  1775
Notice that we deliberately commented out the application of \textit{skew} and
blanchet@33191
  1776
\textit{split}. Let's see if this causes any problems:
blanchet@33191
  1777
blanchet@33191
  1778
\prew
blanchet@35284
  1779
\textbf{theorem}~\textit{wf\_insort\/}:\kern.4em ``$\textit{wf}~t\,\Longrightarrow\, \textit{wf}~(\textit{insort}~t~x)$'' \\
blanchet@33191
  1780
\textbf{nitpick} \\[2\smallskipamount]
blanchet@33191
  1781
\slshape Nitpick found a counterexample for \textit{card} $'a$ = 4: \\[2\smallskipamount]
blanchet@33191
  1782
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@35078
  1783
\hbox{}\qquad\qquad $t = N~a_1~1~\Lambda~\Lambda$ \\
blanchet@35078
  1784
\hbox{}\qquad\qquad $x = a_2$
blanchet@33191
  1785
\postw
blanchet@33191
  1786
blanchet@34038
  1787
It's hard to see why this is a counterexample. To improve readability, we will
blanchet@34038
  1788
restrict the theorem to \textit{nat}, so that we don't need to look up the value
blanchet@34038
  1789
of the $\textit{op}~{<}$ constant to find out which element is smaller than the
blanchet@34038
  1790
other. In addition, we will tell Nitpick to display the value of
blanchet@34038
  1791
$\textit{insort}~t~x$ using the \textit{eval} option. This gives
blanchet@33191
  1792
blanchet@33191
  1793
\prew
blanchet@35284
  1794
\textbf{theorem} \textit{wf\_insort\_nat\/}:\kern.4em ``$\textit{wf}~t\,\Longrightarrow\, \textit{wf}~(\textit{insort}~t~(x\Colon\textit{nat}))$'' \\
blanchet@33191
  1795
\textbf{nitpick} [\textit{eval} = ``$\textit{insort}~t~x$''] \\[2\smallskipamount]
blanchet@33191
  1796
\slshape Nitpick found a counterexample: \\[2\smallskipamount]
blanchet@33191
  1797
\hbox{}\qquad Free variables: \nopagebreak \\
blanchet@33191
  1798
\hbox{}\qquad\qquad $t = N~1~1~\Lambda~\Lambda$ \\
blanchet@33191
  1799
\hbox{}\qquad\qquad $x = 0$ \\
blanchet@33191
  1800
\hbox{}\qquad Evaluated term: \\
blanchet@33191
  1801
\hbox{}\qquad\qquad $\textit{insort}~t~x = N~1~1~(N~0~1~\Lambda~\Lambda)~\Lambda$
blanchet@33191
  1802
\postw
blanchet@33191
  1803
blanchet@33191
  1804
Nitpick's output reveals that the element $0$ was added as a left child of $1$,
blanchet@40147
  1805
where both nodes have a level of 1. This violates the second AA tree invariant,
blanchet@40147
  1806
which states that a left child's level must be less than its parent's. This
blanchet@40147
  1807
shouldn't come as a surprise, considering that we commented out the tree
blanchet@40147
  1808
rebalancing code. Reintroducing the code seems to solve the problem:
blanchet@33191
  1809
blanchet@33191
  1810
\prew
blanchet@35284
  1811
\textbf{theorem}~\textit{wf\_insort\/}:\kern.4em ``$\textit{wf}~t\,\Longrightarrow\, \textit{wf}~(\textit{insort}~t~x)$'' \\
blanchet@33191
  1812
\textbf{nitpick} \\[2\smallskipamount]
blanchet@38183
  1813
{\slshape Nitpick ran out of time after checking 8 of 10 scopes.}
blanchet@33191
  1814
\postw
blanchet@33191
  1815
blanchet@33191
  1816
Insertion should transform the set of elements represented by the tree in the
blanchet@33191
  1817
obvious way:
blanchet@33191
  1818
blanchet@33191
  1819
\prew
blanchet@35284
  1820
\textbf{theorem} \textit{dataset\_insort\/}:\kern.4em
blanchet@33191
  1821
``$\textit{dataset}~(\textit{insort}~t~x) = \{x\} \cup \textit{dataset}~t$'' \\
blanchet@33191
  1822
\textbf{nitpick} \\[2\smallskipamount]
blanchet@38183
  1823
{\slshape Nitpick ran out of time after checking 7 of 10 scopes.}
blanchet@33191
  1824
\postw
blanchet@33191
  1825
blanchet@35072
  1826
We could continue like this and sketch a complete theory of AA trees. Once the
blanchet@35072
  1827
definitions and main theorems are in place and have been thoroughly tested using
blanchet@35072
  1828
Nitpick, we could start working on the proofs. Developing theories this way
blanchet@35072
  1829
usually saves time, because faulty theorems and definitions are discovered much
blanchet@35072
  1830
earlier in the process.
blanchet@33191
  1831
blanchet@33191
  1832
\section{Option Reference}
blanchet@33191
  1833
\label{option-reference}
blanchet@33191
  1834
blanchet@33191
  1835
\def\flushitem#1{\item[]\noindent\kern-\leftmargin \textbf{#1}}
blanchet@33191
  1836
\def\qty#1{$\left<\textit{#1}\right>$}
blanchet@33191
  1837
\def\qtybf#1{$\mathbf{\left<\textbf{\textit{#1}}\right>}$}
blanchet@33191
  1838
\def\optrue#1#2{\flushitem{\textit{#1} $\bigl[$= \qtybf{bool}$\bigr]$\quad [\textit{true}]\hfill (neg.: \textit{#2})}\nopagebreak\\[\parskip]}
blanchet@33191
  1839
\def\opfalse#1#2{\flushitem{\textit{#1} $\bigl[$= \qtybf{bool}$\bigr]$\quad [\textit{false}]\hfill (neg.: \textit{#2})}\nopagebreak\\[\parskip]}
blanchet@33191
  1840
\def\opsmart#1#2{\flushitem{\textit{#1} $\bigl[$= \qtybf{bool\_or\_smart}$\bigr]$\quad [\textit{smart}]\hfill (neg.: \textit{#2})}\nopagebreak\\[\parskip]}
blanchet@34982
  1841
\def\opnodefault#1#2{\flushitem{\textit{#1} = \qtybf{#2}} \nopagebreak\\[\parskip]}
blanchet@34982
  1842
\def\opdefault#1#2#3{\flushitem{\textit{#1} = \qtybf{#2}\quad [\textit{#3}]} \nopagebreak\\[\parskip]}
blanchet@34982
  1843
\def\oparg#1#2#3{\flushitem{\textit{#1} \qtybf{#2} = \qtybf{#3}} \nopagebreak\\[\parskip]}
blanchet@34982
  1844
\def\opargbool#1#2#3{\flushitem{\textit{#1} \qtybf{#2} $\bigl[$= \qtybf{bool}$\bigr]$\hfill (neg.: \textit{#3})}\nopagebreak\\[\parskip]}
blanchet@34982
  1845
\def\opargboolorsmart#1#2#3{\flushitem{\textit{#1} \qtybf{#2} $\bigl[$= \qtybf{bool\_or\_smart}$\bigr]$\hfill (neg.: \textit{#3})}\nopagebreak\\[\parskip]}
blanchet@33191
  1846
blanchet@33191
  1847
Nitpick's behavior can be influenced by various options, which can be specified
blanchet@33191
  1848
in brackets after the \textbf{nitpick} command. Default values can be set
blanchet@33191
  1849
using \textbf{nitpick\_\allowbreak params}. For example:
blanchet@33191
  1850
blanchet@33191
  1851
\prew
blanchet@33191
  1852
\textbf{nitpick\_params} [\textit{verbose}, \,\textit{timeout} = 60$\,s$]
blanchet@33191
  1853
\postw
blanchet@33191
  1854
blanchet@33191
  1855
The options are categorized as follows:\ mode of operation
blanchet@33191
  1856
(\S\ref{mode-of-operation}), scope of search (\S\ref{scope-of-search}), output
blanchet@33191
  1857
format (\S\ref{output-format}), automatic counterexample checks
blanchet@33191
  1858
(\S\ref{authentication}), optimizations
blanchet@33191
  1859
(\S\ref{optimizations}), and timeouts (\S\ref{timeouts}).
blanchet@33191
  1860
blanchet@33561
  1861
You can instruct Nitpick to run automatically on newly entered theorems by
blanchet@33561
  1862
enabling the ``Auto Nitpick'' option from the ``Isabelle'' menu in Proof
blanchet@39317
  1863
General. For automatic runs, \textit{user\_axioms} (\S\ref{mode-of-operation}),
blanchet@39317
  1864
\textit{assms} (\S\ref{mode-of-operation}), and \textit{mono}
blanchet@39317
  1865
(\S\ref{scope-of-search}) are implicitly enabled, \textit{blocking}
blanchet@39317
  1866
(\S\ref{mode-of-operation}), \textit{verbose} (\S\ref{output-format}), and
blanchet@39317
  1867
\textit{debug} (\S\ref{output-format}) are disabled, \textit{max\_threads}
blanchet@39317
  1868
(\S\ref{optimizations}) is taken to be 1, \textit{max\_potential}
blanchet@39317
  1869
(\S\ref{output-format}) is taken to be 0, and \textit{timeout}
blanchet@39317
  1870
(\S\ref{timeouts}) is superseded by the ``Auto Tools Time Limit'' in
blanchet@39317
  1871
Proof General's ``Isabelle'' menu. Nitpick's output is also more concise.
blanchet@33561
  1872
blanchet@33191
  1873
The number of options can be overwhelming at first glance. Do not let that worry
blanchet@33191
  1874
you: Nitpick's defaults have been chosen so that it almost always does the right
blanchet@33191
  1875
thing, and the most important options have been covered in context in
blanchet@35712
  1876
\S\ref{first-steps}.
blanchet@33191
  1877
blanchet@33191
  1878
The descriptions below refer to the following syntactic quantities:
blanchet@33191
  1879
blanchet@33191
  1880
\begin{enum}
blanchet@33191
  1881
\item[$\bullet$] \qtybf{string}: A string.
blanchet@37259
  1882
\item[$\bullet$] \qtybf{string\_list\/}: A space-separated list of strings
blanchet@37259
  1883
(e.g., ``\textit{ichi ni san}'').
blanchet@35284
  1884
\item[$\bullet$] \qtybf{bool\/}: \textit{true} or \textit{false}.
blanchet@35284
  1885
\item[$\bullet$] \qtybf{bool\_or\_smart\/}: \textit{true}, \textit{false}, or \textit{smart}.
blanchet@35284
  1886
\item[$\bullet$] \qtybf{int\/}: An integer. Negative integers are prefixed with a hyphen.
blanchet@35284
  1887
\item[$\bullet$] \qtybf{int\_or\_smart\/}: An integer or \textit{smart}.
blanchet@33191
  1888
\item[$\bullet$] \qtybf{int\_range}: An integer (e.g., 3) or a range
blanchet@33191
  1889
of nonnegative integers (e.g., $1$--$4$). The range symbol `--' can be entered as \texttt{-} (hyphen) or \texttt{\char`\\\char`\<midarrow\char`\>}.
blanchet@33191
  1890
blanchet@33191
  1891
\item[$\bullet$] \qtybf{int\_seq}: A comma-separated sequence of ranges of integers (e.g.,~1{,}3{,}\allowbreak6--8).
blanchet@33191
  1892
\item[$\bullet$] \qtybf{time}: An integer followed by $\textit{min}$ (minutes), $s$ (seconds), or \textit{ms}
blanchet@33191
  1893
(milliseconds), or the keyword \textit{none} ($\infty$ years).
blanchet@35284
  1894
\item[$\bullet$] \qtybf{const\/}: The name of a HOL constant.
blanchet@33191
  1895
\item[$\bullet$] \qtybf{term}: A HOL term (e.g., ``$f~x$'').
blanchet@35284
  1896
\item[$\bullet$] \qtybf{term\_list\/}: A space-separated list of HOL terms (e.g.,
blanchet@33191
  1897
``$f~x$''~``$g~y$'').
blanchet@33191
  1898
\item[$\bullet$] \qtybf{type}: A HOL type.
blanchet@33191
  1899
\end{enum}
blanchet@33191
  1900
blanchet@33191
  1901
Default values are indicated in square brackets. Boolean options have a negated
blanchet@38987
  1902
counterpart (e.g., \textit{blocking} vs.\ \textit{non\_blocking}). When setting
blanchet@33561
  1903
Boolean options, ``= \textit{true}'' may be omitted.
blanchet@33191
  1904
blanchet@33191
  1905
\subsection{Mode of Operation}
blanchet@33191
  1906
\label{mode-of-operation}
blanchet@33191
  1907
blanchet@33191
  1908
\begin{enum}
blanchet@33191
  1909
\optrue{blocking}{non\_blocking}
blanchet@33191
  1910
Specifies whether the \textbf{nitpick} command should operate synchronously.
blanchet@33191
  1911
The asynchronous (non-blocking) mode lets the user start proving the putative
blanchet@33191
  1912
theorem while Nitpick looks for a counterexample, but it can also be more
blanchet@33191
  1913
confusing. For technical reasons, automatic runs currently always block.
blanchet@33191
  1914
blanchet@33191
  1915
\optrue{falsify}{satisfy}
blanchet@33191
  1916
Specifies whether Nitpick should look for falsifying examples (countermodels) or
blanchet@33191
  1917
satisfying examples (models). This manual assumes throughout that
blanchet@33191
  1918
\textit{falsify} is enabled.
blanchet@33191
  1919
blanchet@33191
  1920
\opsmart{user\_axioms}{no\_user\_axioms}
blanchet@33191
  1921
Specifies whether the user-defined axioms (specified using 
blanchet@33191
  1922
\textbf{axiomatization} and \textbf{axioms}) should be considered. If the option
blanchet@33191
  1923
is set to \textit{smart}, Nitpick performs an ad hoc axiom selection based on
blanchet@33191
  1924
the constants that occur in the formula to falsify. The option is implicitly set
blanchet@33191
  1925
to \textit{true} for automatic runs.
blanchet@33191
  1926
blanchet@33191
  1927
\textbf{Warning:} If the option is set to \textit{true}, Nitpick might
blanchet@33191
  1928
nonetheless ignore some polymorphic axioms. Counterexamples generated under
blanchet@35695
  1929
these conditions are tagged as ``quasi genuine.'' The \textit{debug}
blanchet@33191
  1930
(\S\ref{output-format}) option can be used to find out which axioms were
blanchet@33191
  1931
considered.
blanchet@33191
  1932
blanchet@33191
  1933
\nopagebreak
blanchet@33561
  1934
{\small See also \textit{assms} (\S\ref{mode-of-operation}) and \textit{debug}
blanchet@33561
  1935
(\S\ref{output-format}).}
blanchet@33191
  1936
blanchet@33191
  1937
\optrue{assms}{no\_assms}
blanchet@35331
  1938
Specifies whether the relevant assumptions in structured proofs should be
blanchet@33191
  1939
considered. The option is implicitly enabled for automatic runs.
blanchet@33191
  1940
blanchet@33191
  1941
\nopagebreak
blanchet@33561
  1942
{\small See also \textit{user\_axioms} (\S\ref{mode-of-operation}).}
blanchet@33191
  1943
blanchet@33191
  1944
\opfalse{overlord}{no\_overlord}
blanchet@33191
  1945
Specifies whether Nitpick should put its temporary files in
blanchet@33191
  1946
\texttt{\$ISABELLE\_\allowbreak HOME\_\allowbreak USER}, which is useful for
blanchet@33191
  1947
debugging Nitpick but also unsafe if several instances of the tool are run
blanchet@34998
  1948
simultaneously. The files are identified by the extensions
blanchet@34998
  1949
\texttt{.kki}, \texttt{.cnf}, \texttt{.out}, and
blanchet@34998
  1950
\texttt{.err}; you may safely remove them after Nitpick has run.
blanchet@33191
  1951
blanchet@33191
  1952
\nopagebreak
blanchet@33191
  1953
{\small See also \textit{debug} (\S\ref{output-format}).}
blanchet@33191
  1954
\end{enum}
blanchet@33191
  1955
blanchet@33191
  1956
\subsection{Scope of Search}
blanchet@33191
  1957
\label{scope-of-search}
blanchet@33191
  1958
blanchet@33191
  1959
\begin{enum}
blanchet@34982
  1960
\oparg{card}{type}{int\_seq}
blanchet@34124
  1961
Specifies the sequence of cardinalities to use for a given type.
blanchet@33191
  1962
For free types, and often also for \textbf{typedecl}'d types, it usually makes
blanchet@33191
  1963
sense to specify cardinalities as a range of the form \textit{$1$--$n$}.
blanchet@33191
  1964
blanchet@33191
  1965
\nopagebreak
blanchet@35665
  1966
{\small See also \textit{box} (\S\ref{scope-of-search}) and \textit{mono}
blanchet@35665
  1967
(\S\ref{scope-of-search}).}
blanchet@33191
  1968
blanchet@38181
  1969
\opdefault{card}{int\_seq}{$\mathbf{1}$--$\mathbf{10}$}
blanchet@33191
  1970
Specifies the default sequence of cardinalities to use. This can be overridden
blanchet@33191
  1971
on a per-type basis using the \textit{card}~\qty{type} option described above.
blanchet@33191
  1972
blanchet@34982
  1973
\oparg{max}{const}{int\_seq}
blanchet@33191
  1974
Specifies the sequence of maximum multiplicities to use for a given
blanchet@33191
  1975
(co)in\-duc\-tive datatype constructor. A constructor's multiplicity is the
blanchet@33191
  1976
number of distinct values that it can construct. Nonsensical values (e.g.,
blanchet@33191
  1977
\textit{max}~[]~$=$~2) are silently repaired. This option is only available for
blanchet@33191
  1978
datatypes equipped with several constructors.
blanchet@33191
  1979
blanchet@34982
  1980
\opnodefault{max}{int\_seq}
blanchet@33191
  1981
Specifies the default sequence of maximum multiplicities to use for
blanchet@33191
  1982
(co)in\-duc\-tive datatype constructors. This can be overridden on a per-constructor
blanchet@33191
  1983
basis using the \textit{max}~\qty{const} option described above.
blanchet@33191
  1984
blanchet@34124
  1985
\opsmart{binary\_ints}{unary\_ints}
blanchet@34124
  1986
Specifies whether natural numbers and integers should be encoded using a unary
blanchet@34124
  1987
or binary notation. In unary mode, the cardinality fully specifies the subset
blanchet@34124
  1988
used to approximate the type. For example:
blanchet@34124
  1989
%
blanchet@34124
  1990
$$\hbox{\begin{tabular}{@{}rll@{}}%
blanchet@34124
  1991
\textit{card nat} = 4 & induces & $\{0,\, 1,\, 2,\, 3\}$ \\
blanchet@34124
  1992
\textit{card int} = 4 & induces & $\{-1,\, 0,\, +1,\, +2\}$ \\
blanchet@34124
  1993
\textit{card int} = 5 & induces & $\{-2,\, -1,\, 0,\, +1,\, +2\}.$%
blanchet@34124
  1994
\end{tabular}}$$
blanchet@34124
  1995
%
blanchet@34124
  1996
In general:
blanchet@34124
  1997
%
blanchet@34124
  1998
$$\hbox{\begin{tabular}{@{}rll@{}}%
blanchet@34124
  1999
\textit{card nat} = $K$ & induces & $\{0,\, \ldots,\, K - 1\}$ \\
blanchet@34124
  2000
\textit{card int} = $K$ & induces & $\{-\lceil K/2 \rceil + 1,\, \ldots,\, +\lfloor K/2 \rfloor\}.$%
blanchet@34124
  2001
\end{tabular}}$$
blanchet@34124
  2002
%
blanchet@34124
  2003
In binary mode, the cardinality specifies the number of distinct values that can
blanchet@34124
  2004
be constructed. Each of these value is represented by a bit pattern whose length
blanchet@34124
  2005
is specified by the \textit{bits} (\S\ref{scope-of-search}) option. By default,
blanchet@34124
  2006
Nitpick attempts to choose the more appropriate encoding by inspecting the
blanchet@34124
  2007
formula at hand, preferring the binary notation for problems involving
blanchet@34124
  2008
multiplicative operators or large constants.
blanchet@34124
  2009
blanchet@34124
  2010
\textbf{Warning:} For technical reasons, Nitpick always reverts to unary for
blanchet@34124
  2011
problems that refer to the types \textit{rat} or \textit{real} or the constants
blanchet@34126
  2012
\textit{Suc}, \textit{gcd}, or \textit{lcm}.
blanchet@34124
  2013
blanchet@34124
  2014
{\small See also \textit{bits} (\S\ref{scope-of-search}) and
blanchet@34124
  2015
\textit{show\_datatypes} (\S\ref{output-format}).}
blanchet@34124
  2016
blanchet@38181
  2017
\opdefault{bits}{int\_seq}{$\mathbf{1},\mathbf{2},\mathbf{3},\mathbf{4},\mathbf{6},\mathbf{8},\mathbf{10},\mathbf{12},\mathbf{14},\mathbf{16}$}
blanchet@34124
  2018
Specifies the number of bits to use to represent natural numbers and integers in
blanchet@34124
  2019
binary, excluding the sign bit. The minimum is 1 and the maximum is 31.
blanchet@34124
  2020
blanchet@34124
  2021
{\small See also \textit{binary\_ints} (\S\ref{scope-of-search}).}
blanchet@34124
  2022
blanchet@34982
  2023
\opargboolorsmart{wf}{const}{non\_wf}
blanchet@33191
  2024
Specifies whether the specified (co)in\-duc\-tively defined predicate is
blanchet@33191
  2025
well-founded. The option can take the following values:
blanchet@33191
  2026
blanchet@33191
  2027
\begin{enum}
blanchet@36926
  2028
\item[$\bullet$] \textbf{\textit{true}:} Tentatively treat the (co)in\-duc\-tive
blanchet@33191
  2029
predicate as if it were well-founded. Since this is generally not sound when the
blanchet@35695
  2030
predicate is not well-founded, the counterexamples are tagged as ``quasi
blanchet@33191
  2031
genuine.''
blanchet@33191
  2032
blanchet@36926
  2033
\item[$\bullet$] \textbf{\textit{false}:} Treat the (co)in\-duc\-tive predicate
blanchet@33191
  2034
as if it were not well-founded. The predicate is then unrolled as prescribed by
blanchet@33191
  2035
the \textit{star\_linear\_preds}, \textit{iter}~\qty{const}, and \textit{iter}
blanchet@33191
  2036
options.
blanchet@33191
  2037
blanchet@36926
  2038
\item[$\bullet$] \textbf{\textit{smart}:} Try to prove that the inductive
blanchet@33191
  2039
predicate is well-founded using Isabelle's \textit{lexicographic\_order} and
blanchet@34982
  2040
\textit{size\_change} tactics. If this succeeds (or the predicate occurs with an
blanchet@38176
  2041
appropriate polarity in the formula to falsify), use an efficient fixed-point
blanchet@33191
  2042
equation as specification of the predicate; otherwise, unroll the predicates
blanchet@33191
  2043
according to the \textit{iter}~\qty{const} and \textit{iter} options.
blanchet@33191
  2044
\end{enum}
blanchet@33191
  2045
blanchet@33191
  2046
\nopagebreak
blanchet@33191
  2047
{\small See also \textit{iter} (\S\ref{scope-of-search}),
blanchet@33191
  2048
\textit{star\_linear\_preds} (\S\ref{optimizations}), and \textit{tac\_timeout}
blanchet@33191
  2049
(\S\ref{timeouts}).}
blanchet@33191
  2050
blanchet@33191
  2051
\opsmart{wf}{non\_wf}
blanchet@33191
  2052
Specifies the default wellfoundedness setting to use. This can be overridden on
blanchet@33191
  2053
a per-predicate basis using the \textit{wf}~\qty{const} option above.
blanchet@33191
  2054
blanchet@34982
  2055
\oparg{iter}{const}{int\_seq}
blanchet@33191
  2056
Specifies the sequence of iteration counts to use when unrolling a given
blanchet@33191
  2057
(co)in\-duc\-tive predicate. By default, unrolling is applied for inductive
blanchet@33191
  2058
predicates that occur negatively and coinductive predicates that occur
blanchet@33191
  2059
positively in the formula to falsify and that cannot be proved to be
blanchet@33191
  2060
well-founded, but this behavior is influenced by the \textit{wf} option. The
blanchet@33191
  2061
iteration counts are automatically bounded by the cardinality of the predicate's
blanchet@33191
  2062
domain.
blanchet@33191
  2063
blanchet@33191
  2064
{\small See also \textit{wf} (\S\ref{scope-of-search}) and
blanchet@33191
  2065
\textit{star\_linear\_preds} (\S\ref{optimizations}).}
blanchet@33191
  2066
blanchet@38181
  2067
\opdefault{iter}{int\_seq}{$\mathbf{0{,}1{,}2{,}4{,}8{,}12{,}16{,}20{,}24{,}28}$}
blanchet@33191
  2068
Specifies the sequence of iteration counts to use when unrolling (co)in\-duc\-tive
blanchet@33191
  2069
predicates. This can be overridden on a per-predicate basis using the
blanchet@33191
  2070
\textit{iter} \qty{const} option above.
blanchet@33191
  2071
blanchet@38181
  2072
\opdefault{bisim\_depth}{int\_seq}{$\mathbf{9}$}
blanchet@33191
  2073
Specifies the sequence of iteration counts to use when unrolling the
blanchet@33191
  2074
bisimilarity predicate generated by Nitpick for coinductive datatypes. A value
blanchet@33191
  2075
of $-1$ means that no predicate is generated, in which case Nitpick performs an
blanchet@33191
  2076
after-the-fact check to see if the known coinductive datatype values are
blanchet@33191
  2077
bidissimilar. If two values are found to be bisimilar, the counterexample is
blanchet@35695
  2078
tagged as ``quasi genuine.'' The iteration counts are automatically bounded by
blanchet@33191
  2079
the sum of the cardinalities of the coinductive datatypes occurring in the
blanchet@33191
  2080
formula to falsify.
blanchet@33191
  2081
blanchet@34982
  2082
\opargboolorsmart{box}{type}{dont\_box}
blanchet@33191
  2083
Specifies whether Nitpick should attempt to wrap (``box'') a given function or
blanchet@33191
  2084
product type in an isomorphic datatype internally. Boxing is an effective mean
blanchet@33191
  2085
to reduce the search space and speed up Nitpick, because the isomorphic datatype
blanchet@35665
  2086
is approximated by a subset of the possible function or pair values.
blanchet@35665
  2087
Like other drastic optimizations, it can also prevent the discovery of
blanchet@33191
  2088
counterexamples. The option can take the following values:
blanchet@33191
  2089
blanchet@33191
  2090
\begin{enum}
blanchet@36926
  2091
\item[$\bullet$] \textbf{\textit{true}:} Box the specified type whenever
blanchet@33191
  2092
practicable.
blanchet@36926
  2093
\item[$\bullet$] \textbf{\textit{false}:} Never box the type.
blanchet@36926
  2094
\item[$\bullet$] \textbf{\textit{smart}:} Box the type only in contexts where it
blanchet@33191
  2095
is likely to help. For example, $n$-tuples where $n > 2$ and arguments to
blanchet@33191
  2096
higher-order functions are good candidates for boxing.
blanchet@33191
  2097
\end{enum}
blanchet@33191
  2098
blanchet@33191
  2099
\nopagebreak
blanchet@35665
  2100
{\small See also \textit{finitize} (\S\ref{scope-of-search}), \textit{verbose}
blanchet@35665
  2101
(\S\ref{output-format}), and \textit{debug} (\S\ref{output-format}).}
blanchet@33191
  2102
blanchet@33191
  2103
\opsmart{box}{dont\_box}
blanchet@33191
  2104
Specifies the default boxing setting to use. This can be overridden on a
blanchet@33191
  2105
per-type basis using the \textit{box}~\qty{type} option described above.
blanchet@33191
  2106
blanchet@35665
  2107
\opargboolorsmart{finitize}{type}{dont\_finitize}
blanchet@35665
  2108
Specifies whether Nitpick should attempt to finitize a given type, which can be
blanchet@35665
  2109
a function type or an infinite ``shallow datatype'' (an infinite datatype whose
blanchet@35665
  2110
constructors don't appear in the problem).
blanchet@35665
  2111
blanchet@35665
  2112
For function types, Nitpick performs a monotonicity analysis to detect functions
blanchet@35665
  2113
that are constant at all but finitely many points (e.g., finite sets) and treats
blanchet@35665
  2114
such occurrences specially, thereby increasing the precision. The option can
blanchet@35665
  2115
then take the following values:
blanchet@35665
  2116
blanchet@35665
  2117
\begin{enum}
blanchet@36926
  2118
\item[$\bullet$] \textbf{\textit{false}:} Don't attempt to finitize the type.
blanchet@36926
  2119
\item[$\bullet$] \textbf{\textit{true}} or \textbf{\textit{smart}:} Finitize the
blanchet@35665
  2120
type wherever possible.
blanchet@35665
  2121
\end{enum}
blanchet@35665
  2122
blanchet@35665
  2123
The semantics of the option is somewhat different for infinite shallow
blanchet@35665
  2124
datatypes:
blanchet@35665
  2125
blanchet@35665
  2126
\begin{enum}
blanchet@36926
  2127
\item[$\bullet$] \textbf{\textit{true}:} Finitize the datatype. Since this is
blanchet@35695
  2128
unsound, counterexamples generated under these conditions are tagged as ``quasi
blanchet@35665
  2129
genuine.''
blanchet@36926
  2130
\item[$\bullet$] \textbf{\textit{false}:} Don't attempt to finitize the datatype.
blanchet@36926
  2131
\item[$\bullet$] \textbf{\textit{smart}:} Perform a monotonicity analysis to
blanchet@35665
  2132
detect whether the datatype can be safely finitized before finitizing it.
blanchet@35665
  2133
\end{enum}
blanchet@35665
  2134
blanchet@35665
  2135
Like other drastic optimizations, finitization can sometimes prevent the
blanchet@35665
  2136
discovery of counterexamples.
blanchet@35665
  2137
blanchet@35665
  2138
\nopagebreak
blanchet@35665
  2139
{\small See also \textit{box} (\S\ref{scope-of-search}), \textit{mono}
blanchet@35665
  2140
(\S\ref{scope-of-search}), \textit{verbose} (\S\ref{output-format}), and
blanchet@35665
  2141
\textit{debug} (\S\ref{output-format}).}
blanchet@35665
  2142
blanchet@35665
  2143
\opsmart{finitize}{dont\_finitize}
blanchet@35665
  2144
Specifies the default finitization setting to use. This can be overridden on a
blanchet@35665
  2145
per-type basis using the \textit{finitize}~\qty{type} option described above.
blanchet@35665
  2146
blanchet@34982
  2147
\opargboolorsmart{mono}{type}{non\_mono}
blanchet@35665
  2148
Specifies whether the given type should be considered monotonic when enumerating
blanchet@35665
  2149
scopes and finitizing types. If the option is set to \textit{smart}, Nitpick
blanchet@35665
  2150
performs a monotonicity check on the type. Setting this option to \textit{true}
blanchet@35665
  2151
can reduce the number of scopes tried, but it can also diminish the chance of
blanchet@39317
  2152
finding a counterexample, as demonstrated in \S\ref{scope-monotonicity}. The
blanchet@39317
  2153
option is implicitly set to \textit{true} for automatic runs.
blanchet@33191
  2154
blanchet@33191
  2155
\nopagebreak
blanchet@33191
  2156
{\small See also \textit{card} (\S\ref{scope-of-search}),
blanchet@35665
  2157
\textit{finitize} (\S\ref{scope-of-search}),
blanchet@33556
  2158
\textit{merge\_type\_vars} (\S\ref{scope-of-search}), and \textit{verbose}
blanchet@33191
  2159
(\S\ref{output-format}).}
blanchet@33191
  2160
blanchet@35665
  2161
\opsmart{mono}{non\_mono}
blanchet@33191
  2162
Specifies the default monotonicity setting to use. This can be overridden on a
blanchet@33191
  2163
per-type basis using the \textit{mono}~\qty{type} option described above.
blanchet@33191
  2164
blanchet@33556
  2165
\opfalse{merge\_type\_vars}{dont\_merge\_type\_vars}
blanchet@33191
  2166
Specifies whether type variables with the same sort constraints should be
blanchet@33191
  2167
merged. Setting this option to \textit{true} can reduce the number of scopes
blanchet@33191
  2168
tried and the size of the generated Kodkod formulas, but it also diminishes the
blanchet@33191
  2169
theoretical chance of finding a counterexample.
blanchet@33191
  2170
blanchet@33191
  2171
{\small See also \textit{mono} (\S\ref{scope-of-search}).}
blanchet@34982
  2172
blanchet@34982
  2173
\opargbool{std}{type}{non\_std}
blanchet@35189
  2174
Specifies whether the given (recursive) datatype should be given standard
blanchet@35189
  2175
models. Nonstandard models are unsound but can help debug structural induction
blanchet@35189
  2176
proofs, as explained in \S\ref{inductive-properties}.
blanchet@34982
  2177
blanchet@34982
  2178
\optrue{std}{non\_std}
blanchet@34982
  2179
Specifies the default standardness to use. This can be overridden on a per-type
blanchet@34982
  2180
basis using the \textit{std}~\qty{type} option described above.
blanchet@33191
  2181
\end{enum}
blanchet@33191
  2182
blanchet@33191
  2183
\subsection{Output Format}
blanchet@33191
  2184
\label{output-format}
blanchet@33191
  2185
blanchet@33191
  2186
\begin{enum}
blanchet@33191
  2187
\opfalse{verbose}{quiet}
blanchet@33191
  2188
Specifies whether the \textbf{nitpick} command should explain what it does. This
blanchet@33191
  2189
option is useful to determine which scopes are tried or which SAT solver is
blanchet@33191
  2190
used. This option is implicitly disabled for automatic runs.
blanchet@33191
  2191
blanchet@33191
  2192
\opfalse{debug}{no\_debug}
blanchet@33191
  2193
Specifies whether Nitpick should display additional debugging information beyond
blanchet@33191
  2194
what \textit{verbose} already displays. Enabling \textit{debug} also enables
blanchet@33191
  2195
\textit{verbose} and \textit{show\_all} behind the scenes. The \textit{debug}
blanchet@33191
  2196
option is implicitly disabled for automatic runs.
blanchet@33191
  2197
blanchet@33191
  2198
\nopagebreak
blanchet@33561
  2199
{\small See also \textit{overlord} (\S\ref{mode-of-operation}) and
blanchet@33561
  2200
\textit{batch\_size} (\S\ref{optimizations}).}
blanchet@33191
  2201
blanchet@33191
  2202
\opfalse{show\_datatypes}{hide\_datatypes}
blanchet@33191
  2203
Specifies whether the subsets used to approximate (co)in\-duc\-tive datatypes should
blanchet@33191
  2204
be displayed as part of counterexamples. Such subsets are sometimes helpful when
blanchet@33191
  2205
investigating whether a potential counterexample is genuine or spurious, but
blanchet@33191
  2206
their potential for clutter is real.
blanchet@33191
  2207
blanchet@33191
  2208
\opfalse{show\_consts}{hide\_consts}
blanchet@33191
  2209
Specifies whether the values of constants occurring in the formula (including
blanchet@33191
  2210
its axioms) should be displayed along with any counterexample. These values are
blanchet@33191
  2211
sometimes helpful when investigating why a counterexample is
blanchet@33191
  2212
genuine, but they can clutter the output.
blanchet@33191
  2213
blanchet@37169
  2214
\opnodefault{show\_all}{bool}
blanchet@37169
  2215
Abbreviation for \textit{show\_datatypes} and \textit{show\_consts}.
blanchet@33191
  2216
blanchet@34982
  2217
\opdefault{max\_potential}{int}{$\mathbf{1}$}
blanchet@33191
  2218
Specifies the maximum number of potential counterexamples to display. Setting
blanchet@33191
  2219
this option to 0 speeds up the search for a genuine counterexample. This option
blanchet@33191
  2220
is implicitly set to 0 for automatic runs. If you set this option to a value
blanchet@35710
  2221
greater than 1, you will need an incremental SAT solver, such as
blanchet@35710
  2222
\textit{MiniSat\_JNI} (recommended) and \textit{SAT4J}. Be aware that many of
blanchet@35710
  2223
the counterexamples may be identical.
blanchet@33191
  2224
blanchet@33191
  2225
\nopagebreak
blanchet@33561
  2226
{\small See also \textit{check\_potential} (\S\ref{authentication}) and
blanchet@33191
  2227
\textit{sat\_solver} (\S\ref{optimizations}).}
blanchet@33191
  2228
blanchet@34982
  2229
\opdefault{max\_genuine}{int}{$\mathbf{1}$}
blanchet@33191
  2230
Specifies the maximum number of genuine counterexamples to display. If you set
blanchet@35710
  2231
this option to a value greater than 1, you will need an incremental SAT solver,
blanchet@35710
  2232
such as \textit{MiniSat\_JNI} (recommended) and \textit{SAT4J}. Be aware that
blanchet@35710
  2233
many of the counterexamples may be identical.
blanchet@33191
  2234
blanchet@33191
  2235
\nopagebreak
blanchet@33191
  2236
{\small See also \textit{check\_genuine} (\S\ref{authentication}) and
blanchet@33191
  2237
\textit{sat\_solver} (\S\ref{optimizations}).}
blanchet@33191
  2238
blanchet@34982
  2239
\opnodefault{eval}{term\_list}
blanchet@33191
  2240
Specifies the list of terms whose values should be displayed along with
blanchet@33191
  2241
counterexamples. This option suffers from an ``observer effect'': Nitpick might
blanchet@33191
  2242
find different counterexamples for different values of this option.
blanchet@33191
  2243
blanchet@37259
  2244
\oparg{atoms}{type}{string\_list}
blanchet@37259
  2245
Specifies the names to use to refer to the atoms of the given type. By default,
blanchet@37259
  2246
Nitpick generates names of the form $a_1, \ldots, a_n$, where $a$ is the first
blanchet@37259
  2247
letter of the type's name.
blanchet@37259
  2248
blanchet@37259
  2249
\opnodefault{atoms}{string\_list}
blanchet@37259
  2250
Specifies the default names to use to refer to atoms of any type. For example,
blanchet@37259
  2251
to call the three atoms of type ${'}a$ \textit{ichi}, \textit{ni}, and
blanchet@37259
  2252
\textit{san} instead of $a_1$, $a_2$, $a_3$, specify the option
blanchet@37259
  2253
``\textit{atoms}~${'}a$ = \textit{ichi~ni~san}''. The default names can be
blanchet@37259
  2254
overridden on a per-type basis using the \textit{atoms}~\qty{type} option
blanchet@37259
  2255
described above.
blanchet@37259
  2256
blanchet@34982
  2257
\oparg{format}{term}{int\_seq}
blanchet@33191
  2258
Specifies how to uncurry the value displayed for a variable or constant.
blanchet@33191
  2259
Uncurrying sometimes increases the readability of the output for high-arity
blanchet@33191
  2260
functions. For example, given the variable $y \mathbin{\Colon} {'a}\Rightarrow
blanchet@33191
  2261
{'b}\Rightarrow {'c}\Rightarrow {'d}\Rightarrow {'e}\Rightarrow {'f}\Rightarrow
blanchet@33191
  2262
{'g}$, setting \textit{format}~$y$ = 3 tells Nitpick to group the last three
blanchet@33191
  2263
arguments, as if the type had been ${'a}\Rightarrow {'b}\Rightarrow
blanchet@33191
  2264
{'c}\Rightarrow {'d}\times {'e}\times {'f}\Rightarrow {'g}$. In general, a list
blanchet@33191
  2265
of values $n_1,\ldots,n_k$ tells Nitpick to show the last $n_k$ arguments as an
blanchet@33191
  2266
$n_k$-tuple, the previous $n_{k-1}$ arguments as an $n_{k-1}$-tuple, and so on;
blanchet@33191
  2267
arguments that are not accounted for are left alone, as if the specification had
blanchet@33191
  2268
been $1,\ldots,1,n_1,\ldots,n_k$.
blanchet@33191
  2269
blanchet@34982
  2270
\opdefault{format}{int\_seq}{$\mathbf{1}$}
blanchet@33191
  2271
Specifies the default format to use. Irrespective of the default format, the
blanchet@33191
  2272
extra arguments to a Skolem constant corresponding to the outer bound variables
blanchet@33191
  2273
are kept separated from the remaining arguments, the \textbf{for} arguments of
blanchet@33191
  2274
an inductive definitions are kept separated from the remaining arguments, and
blanchet@33191
  2275
the iteration counter of an unrolled inductive definition is shown alone. The
blanchet@33191
  2276
default format can be overridden on a per-variable or per-constant basis using
blanchet@33191
  2277
the \textit{format}~\qty{term} option described above.
blanchet@33191
  2278
\end{enum}
blanchet@33191
  2279
blanchet@33191
  2280
\subsection{Authentication}
blanchet@33191
  2281
\label{authentication}
blanchet@33191
  2282
blanchet@33191
  2283
\begin{enum}
blanchet@33191
  2284
\opfalse{check\_potential}{trust\_potential}
blanchet@33191
  2285
Specifies whether potential counterexamples should be given to Isabelle's
blanchet@33191
  2286
\textit{auto} tactic to assess their validity. If a potential counterexample is
blanchet@33191
  2287
shown to be genuine, Nitpick displays a message to this effect and terminates.
blanchet@33191
  2288
blanchet@33191
  2289
\nopagebreak
blanchet@33561
  2290
{\small See also \textit{max\_potential} (\S\ref{output-format}).}
blanchet@33191
  2291
blanchet@33191
  2292
\opfalse{check\_genuine}{trust\_genuine}
blanchet@35695
  2293
Specifies whether genuine and quasi genuine counterexamples should be given to
blanchet@33191
  2294
Isabelle's \textit{auto} tactic to assess their validity. If a ``genuine''
blanchet@33191
  2295
counterexample is shown to be spurious, the user is kindly asked to send a bug
blanchet@33191
  2296
report to the author at
blanchet@33191
  2297
\texttt{blan{\color{white}nospam}\kern-\wd\boxA{}chette@in.tum.de}.
blanchet@33191
  2298
blanchet@33191
  2299
\nopagebreak
blanchet@33561
  2300
{\small See also \textit{max\_genuine} (\S\ref{output-format}).}
blanchet@33191
  2301
blanchet@34982
  2302
\opnodefault{expect}{string}
blanchet@33191
  2303
Specifies the expected outcome, which must be one of the following:
blanchet@33191
  2304
blanchet@33191
  2305
\begin{enum}
blanchet@36926
  2306
\item[$\bullet$] \textbf{\textit{genuine}:} Nitpick found a genuine counterexample.
blanchet@36926
  2307
\item[$\bullet$] \textbf{\textit{quasi\_genuine}:} Nitpick found a ``quasi
blanchet@33191
  2308
genuine'' counterexample (i.e., a counterexample that is genuine unless
blanchet@33191
  2309
it contradicts a missing axiom or a dangerous option was used inappropriately).
blanchet@36926
  2310
\item[$\bullet$] \textbf{\textit{potential}:} Nitpick found a potential counterexample.
blanchet@36926
  2311
\item[$\bullet$] \textbf{\textit{none}:} Nitpick found no counterexample.
blanchet@36926
  2312
\item[$\bullet$] \textbf{\textit{unknown}:} Nitpick encountered some problem (e.g.,
blanchet@33191
  2313
Kodkod ran out of memory).
blanchet@33191
  2314
\end{enum}
blanchet@33191
  2315
blanchet@33191
  2316
Nitpick emits an error if the actual outcome differs from the expected outcome.
blanchet@33191
  2317
This option is useful for regression testing.
blanchet@33191
  2318
\end{enum}
blanchet@33191
  2319
blanchet@33191
  2320
\subsection{Optimizations}
blanchet@33191
  2321
\label{optimizations}
blanchet@33191
  2322
blanchet@33191
  2323
\def\cpp{C\nobreak\raisebox{.1ex}{+}\nobreak\raisebox{.1ex}{+}}
blanchet@33191
  2324
blanchet@33191
  2325
\sloppy
blanchet@33191
  2326
blanchet@33191
  2327
\begin{enum}
blanchet@34982
  2328
\opdefault{sat\_solver}{string}{smart}
blanchet@33191
  2329
Specifies which SAT solver to use. SAT solvers implemented in C or \cpp{} tend
blanchet@33191
  2330
to be faster than their Java counterparts, but they can be more difficult to
blanchet@33191
  2331
install. Also, if you set the \textit{max\_potential} (\S\ref{output-format}) or
blanchet@33191
  2332
\textit{max\_genuine} (\S\ref{output-format}) option to a value greater than 1,
blanchet@35078
  2333
you will need an incremental SAT solver, such as \textit{MiniSat\_JNI}
blanchet@33191
  2334
(recommended) or \textit{SAT4J}.
blanchet@33191
  2335
blanchet@33191
  2336
The supported solvers are listed below:
blanchet@33191
  2337
blanchet@33191
  2338
\begin{enum}
blanchet@33191
  2339
blanchet@38125
  2340
\item[$\bullet$] \textbf{\textit{MiniSat}:} MiniSat is an efficient solver
blanchet@38125
  2341
written in \cpp{}. To use MiniSat, set the environment variable
blanchet@38125
  2342
\texttt{MINISAT\_HOME} to the directory that contains the \texttt{minisat}
blanchet@38122
  2343
executable.%
blanchet@38122
  2344
\footnote{Important note for Cygwin users: The path must be specified using
blanchet@38122
  2345
native Windows syntax. Make sure to escape backslashes properly.%
blanchet@38122
  2346
\label{cygwin-paths}}
blanchet@35695
  2347
The \cpp{} sources and executables for MiniSat are available at
blanchet@33191
  2348
\url{http://minisat.se/MiniSat.html}. Nitpick has been tested with versions 1.14
blanchet@33191
  2349
and 2.0 beta (2007-07-21).
blanchet@33191
  2350
blanchet@36926
  2351
\item[$\bullet$] \textbf{\textit{MiniSat\_JNI}:} The JNI (Java Native Interface)
blanchet@35710
  2352
version of MiniSat is bundled with Kodkodi and is precompiled for the major
blanchet@35710
  2353
platforms. It is also available from \texttt{native\-solver.\allowbreak tgz},
blanchet@35710
  2354
which you will find on Kodkod's web site \cite{kodkod-2009}. Unlike the standard
blanchet@33191
  2355
version of MiniSat, the JNI version can be used incrementally.
blanchet@33191
  2356
blanchet@38125
  2357
\item[$\bullet$] \textbf{\textit{CryptoMiniSat}:} CryptoMiniSat is the winner of
blanchet@38125
  2358
the 2010 SAT Race. To use CryptoMiniSat, set the environment variable
blanchet@38125
  2359
\texttt{CRYPTO\-MINISAT\_}\discretionary{}{}{}\texttt{HOME} to the directory that contains the \texttt{crypto\-minisat}
blanchet@38125
  2360
executable.%
blanchet@38125
  2361
\footref{cygwin-paths}
blanchet@38125
  2362
The \cpp{} sources and executables for Crypto\-Mini\-Sat are available at
blanchet@38125
  2363
\url{http://planete.inrialpes.fr/~soos/}\allowbreak\url{CryptoMiniSat2/index.php}.
blanchet@38125
  2364
Nitpick has been tested with version 2.51.
blanchet@38125
  2365
blanchet@36926
  2366
\item[$\bullet$] \textbf{\textit{PicoSAT}:} PicoSAT is an efficient solver
blanchet@33731
  2367
written in C. You can install a standard version of
blanchet@33191
  2368
PicoSAT and set the environment variable \texttt{PICOSAT\_HOME} to the directory
blanchet@35695
  2369
that contains the \texttt{picosat} executable.%
blanchet@35695
  2370
\footref{cygwin-paths}
blanchet@35695
  2371
The C sources for PicoSAT are
blanchet@33191
  2372
available at \url{http://fmv.jku.at/picosat/} and are also bundled with Kodkodi.
blanchet@33191
  2373
Nitpick has been tested with version 913.
blanchet@33191
  2374
blanchet@36926
  2375
\item[$\bullet$] \textbf{\textit{zChaff}:} zChaff is an efficient solver written
blanchet@33191
  2376
in \cpp{}. To use zChaff, set the environment variable \texttt{ZCHAFF\_HOME} to
blanchet@35695
  2377
the directory that contains the \texttt{zchaff} executable.%
blanchet@35695
  2378
\footref{cygwin-paths}
blanchet@35695
  2379
The \cpp{} sources and executables for zChaff are available at
blanchet@33191
  2380
\url{http://www.princeton.edu/~chaff/zchaff.html}. Nitpick has been tested with
blanchet@33191
  2381
versions 2004-05-13, 2004-11-15, and 2007-03-12.
blanchet@33191
  2382
blanchet@36926
  2383
\item[$\bullet$] \textbf{\textit{zChaff\_JNI}:} The JNI version of zChaff is
blanchet@35710
  2384
bundled with Kodkodi and is precompiled for the major
blanchet@35710
  2385
platforms. It is also available from \texttt{native\-solver.\allowbreak tgz},
blanchet@35710
  2386
which you will find on Kodkod's web site \cite{kodkod-2009}.
blanchet@33191
  2387
blanchet@36926
  2388
\item[$\bullet$] \textbf{\textit{RSat}:} RSat is an efficient solver written in
blanchet@33191
  2389
\cpp{}. To use RSat, set the environment variable \texttt{RSAT\_HOME} to the
blanchet@35695
  2390
directory that contains the \texttt{rsat} executable.%
blanchet@35695
  2391
\footref{cygwin-paths}
blanchet@35695
  2392
The \cpp{} sources for RSat are available at
blanchet@35695
  2393
\url{http://reasoning.cs.ucla.edu/rsat/}. Nitpick has been tested with version
blanchet@35695
  2394
2.01.
blanchet@33191
  2395
blanchet@36926
  2396
\item[$\bullet$] \textbf{\textit{BerkMin}:} BerkMin561 is an efficient solver
blanchet@33191
  2397
written in C. To use BerkMin, set the environment variable
blanchet@33191
  2398
\texttt{BERKMIN\_HOME} to the directory that contains the \texttt{BerkMin561}
blanchet@35695
  2399
executable.\footref{cygwin-paths}
blanchet@35695
  2400
The BerkMin executables are available at
blanchet@33191
  2401
\url{http://eigold.tripod.com/BerkMin.html}.
blanchet@33191
  2402
blanchet@36926
  2403
\item[$\bullet$] \textbf{\textit{BerkMin\_Alloy}:} Variant of BerkMin that is
blanchet@33191
  2404
included with Alloy 4 and calls itself ``sat56'' in its banner text. To use this
blanchet@33191
  2405
version of BerkMin, set the environment variable
blanchet@33191
  2406
\texttt{BERKMINALLOY\_HOME} to the directory that contains the \texttt{berkmin}
blanchet@35695
  2407
executable.%
blanchet@35695
  2408
\footref{cygwin-paths}
blanchet@33191
  2409
blanchet@36926
  2410
\item[$\bullet$] \textbf{\textit{Jerusat}:} Jerusat 1.3 is an efficient solver
blanchet@33191
  2411
written in C. To use Jerusat, set the environment variable
blanchet@33191
  2412
\texttt{JERUSAT\_HOME} to the directory that contains the \texttt{Jerusat1.3}
blanchet@35695
  2413
executable.%
blanchet@35695
  2414
\footref{cygwin-paths}
blanchet@35695
  2415
The C sources for Jerusat are available at
blanchet@33191
  2416
\url{http://www.cs.tau.ac.il/~ale1/Jerusat1.3.tgz}.
blanchet@33191
  2417
blanchet@36926
  2418
\item[$\bullet$] \textbf{\textit{SAT4J}:} SAT4J is a reasonably efficient solver
blanchet@33191
  2419
written in Java that can be used incrementally. It is bundled with Kodkodi and
blanchet@33191
  2420
requires no further installation or configuration steps. Do not attempt to
blanchet@33191
  2421
install the official SAT4J packages, because their API is incompatible with
blanchet@33191
  2422
Kodkod.
blanchet@33191
  2423
blanchet@36926
  2424
\item[$\bullet$] \textbf{\textit{SAT4J\_Light}:} Variant of SAT4J that is
blanchet@33191
  2425
optimized for small problems. It can also be used incrementally.
blanchet@33191
  2426
blanchet@36926
  2427
\item[$\bullet$] \textbf{\textit{smart}:} If \textit{sat\_solver} is set to
blanchet@38125
  2428
\textit{smart}, Nitpick selects the first solver among the above that is
blanchet@38125
  2429
recognized by Isabelle. If \textit{verbose} (\S\ref{output-format}) is enabled,
blanchet@38125
  2430
Nitpick displays which SAT solver was chosen.
blanchet@33191
  2431
\end{enum}
blanchet@33191
  2432
\fussy
blanchet@33191
  2433
blanchet@34982
  2434
\opdefault{batch\_size}{int\_or\_smart}{smart}
blanchet@33191
  2435
Specifies the maximum number of Kodkod problems that should be lumped together
blanchet@33191
  2436
when invoking Kodkodi. Each problem corresponds to one scope. Lumping problems
blanchet@33191
  2437
together ensures that Kodkodi is launched less often, but it makes the verbose
blanchet@33191
  2438
output less readable and is sometimes detrimental to performance. If
blanchet@33191
  2439
\textit{batch\_size} is set to \textit{smart}, the actual value used is 1 if
blanchet@38181
  2440
\textit{debug} (\S\ref{output-format}) is set and 50 otherwise.
blanchet@33191
  2441
blanchet@33191
  2442
\optrue{destroy\_constrs}{dont\_destroy\_constrs}
blanchet@33191
  2443
Specifies whether formulas involving (co)in\-duc\-tive datatype constructors should
blanchet@33191
  2444
be rewritten to use (automatically generated) discriminators and destructors.
blanchet@33191
  2445
This optimization can drastically reduce the size of the Boolean formulas given
blanchet@33191
  2446
to the SAT solver.
blanchet@33191
  2447
blanchet@33191
  2448
\nopagebreak
blanchet@33191
  2449
{\small See also \textit{debug} (\S\ref{output-format}).}
blanchet@33191
  2450
blanchet@33191
  2451
\optrue{specialize}{dont\_specialize}
blanchet@33191
  2452
Specifies whether functions invoked with static arguments should be specialized.
blanchet@33191
  2453
This optimization can drastically reduce the search space, especially for
blanchet@33191
  2454
higher-order functions.
blanchet@33191
  2455
blanchet@33191
  2456
\nopagebreak
blanchet@33191
  2457
{\small See also \textit{debug} (\S\ref{output-format}) and
blanchet@33191
  2458
\textit{show\_consts} (\S\ref{output-format}).}
blanchet@33191
  2459
blanchet@33191
  2460
\optrue{star\_linear\_preds}{dont\_star\_linear\_preds}
blanchet@33191
  2461
Specifies whether Nitpick should use Kodkod's transitive closure operator to
blanchet@33191
  2462
encode non-well-founded ``linear inductive predicates,'' i.e., inductive
blanchet@33191
  2463
predicates for which each the predicate occurs in at most one assumption of each
blanchet@33191
  2464
introduction rule. Using the reflexive transitive closure is in principle
blanchet@33191
  2465
equivalent to setting \textit{iter} to the cardinality of the predicate's
blanchet@33191
  2466
domain, but it is usually more efficient.
blanchet@33191
  2467
blanchet@33191
  2468
{\small See also \textit{wf} (\S\ref{scope-of-search}), \textit{debug}
blanchet@33191
  2469
(\S\ref{output-format}), and \textit{iter} (\S\ref{scope-of-search}).}
blanchet@33191
  2470
blanchet@33191
  2471
{\small See also \textit{debug} (\S\ref{output-format}).}
blanchet@33191
  2472
blanchet@38209
  2473
\opnodefault{whack}{term\_list}
blanchet@38209
  2474
Specifies a list of atomic terms (usually constants, but also free and schematic
blanchet@38209
  2475
variables) that should be taken as being $\unk$ (unknown). This can be useful to
blanchet@38209
  2476
reduce the size of the Kodkod problem if you can guess in advance that a
blanchet@38209
  2477
constant might not be needed to find a countermodel.
blanchet@38209
  2478
blanchet@38209
  2479
{\small See also \textit{debug} (\S\ref{output-format}).}
blanchet@38209
  2480
blanchet@33191
  2481
\optrue{peephole\_optim}{no\_peephole\_optim}
blanchet@33191
  2482
Specifies whether Nitpick should simplify the generated Kodkod formulas using a
blanchet@33191
  2483
peephole optimizer. These optimizations can make a significant difference.
blanchet@33191
  2484
Unless you are tracking down a bug in Nitpick or distrust the peephole
blanchet@33191
  2485
optimizer, you should leave this option enabled.
blanchet@33191
  2486
blanchet@38128
  2487
\opdefault{datatype\_sym\_break}{int}{5}
blanchet@38128
  2488
Specifies an upper bound on the number of datatypes for which Nitpick generates
blanchet@38128
  2489
symmetry breaking predicates. Symmetry breaking can speed up the SAT solver
blanchet@38128
  2490
considerably, especially for unsatisfiable problems, but too much of it can slow
blanchet@38128
  2491
it down.
blanchet@38128
  2492
blanchet@38128
  2493
\opdefault{kodkod\_sym\_break}{int}{15}
blanchet@38128
  2494
Specifies an upper bound on the number of relations for which Kodkod generates
blanchet@38128
  2495
symmetry breaking predicates. Symmetry breaking can speed up the SAT solver
blanchet@38128
  2496
considerably, especially for unsatisfiable problems, but too much of it can slow
blanchet@38128
  2497
it down.
blanchet@38128
  2498
blanchet@34982
  2499
\opdefault{max\_threads}{int}{0}
blanchet@33191
  2500
Specifies the maximum number of threads to use in Kodkod. If this option is set
blanchet@33191
  2501
to 0, Kodkod will compute an appropriate value based on the number of processor
blanchet@39317
  2502
cores available. The option is implicitly set to 1 for automatic runs.
blanchet@33191
  2503
blanchet@33191
  2504
\nopagebreak
blanchet@33191
  2505
{\small See also \textit{batch\_size} (\S\ref{optimizations}) and
blanchet@33191
  2506
\textit{timeout} (\S\ref{timeouts}).}
blanchet@33191
  2507
\end{enum}
blanchet@33191
  2508
blanchet@33191
  2509
\subsection{Timeouts}
blanchet@33191
  2510
\label{timeouts}
blanchet@33191
  2511
blanchet@33191
  2512
\begin{enum}
blanchet@34982
  2513
\opdefault{timeout}{time}{$\mathbf{30}$ s}
blanchet@33191
  2514
Specifies the maximum amount of time that the \textbf{nitpick} command should
blanchet@33191
  2515
spend looking for a counterexample. Nitpick tries to honor this constraint as
blanchet@33191
  2516
well as it can but offers no guarantees. For automatic runs,
blanchet@33561
  2517
\textit{timeout} is ignored; instead, Auto Quickcheck and Auto Nitpick share
blanchet@33561
  2518
a time slot whose length is specified by the ``Auto Counterexample Time
blanchet@33561
  2519
Limit'' option in Proof General.
blanchet@33191
  2520
blanchet@33191
  2521
\nopagebreak
blanchet@33561
  2522
{\small See also \textit{max\_threads} (\S\ref{optimizations}).}
blanchet@33191
  2523
blanchet@34982
  2524
\opdefault{tac\_timeout}{time}{$\mathbf{500}$\,ms}
blanchet@33191
  2525
Specifies the maximum amount of time that the \textit{auto} tactic should use
blanchet@33191
  2526
when checking a counterexample, and similarly that \textit{lexicographic\_order}
blanchet@34982
  2527
and \textit{size\_change} should use when checking whether a (co)in\-duc\-tive
blanchet@33191
  2528
predicate is well-founded. Nitpick tries to honor this constraint as well as it
<