
\chapter{Basic Isar Language Elements}\label{ch:pure-syntax}

Subsequently, we introduce the main part of the basic Isar theory and proof
commands as provided by Isabelle/Pure.  Chapter~\ref{ch:gen-tools} describes
further Isar elements provided by generic tools and packages (such as the
Simplifier) that are either part of Pure Isabelle or pre-loaded by most object
logics.  Chapter~\ref{ch:hol-tools} refers to actual object-logic specific
elements of Isabelle/HOL.

\medskip

Isar commands may be either \emph{proper} document constructors, or
\emph{improper commands}.  Some proof methods and attributes introduced later
are classified as improper as well.  Improper Isar language elements, which
are subsequently marked by $^*$, are often helpful when developing proof
documents, while their use is discouraged for the final outcome.  Typical
examples are diagnostic commands that print terms or theorems according to the
current context; other commands even emulate old-style tactical theorem
proving, which facilitates porting of legacy proof scripts.


\section{Theory commands}

\subsection{Defining theories}\label{sec:begin-thy}

\indexisarcmd{header}\indexisarcmd{theory}\indexisarcmd{end}\indexisarcmd{context}
\begin{matharray}{rcl}
  \isarcmd{header} & : & \isarkeep{toplevel} \\
  \isarcmd{theory} & : & \isartrans{\cdot}{theory} \\
  \isarcmd{context}^* & : & \isartrans{\cdot}{theory} \\
  \isarcmd{end} & : & \isartrans{theory}{\cdot} \\
\end{matharray}

Isabelle/Isar ``new-style'' theories are either defined via theory files or
interactively.  Both theory-level specifications and proofs are handled
uniformly --- occasionally definitional mechanisms even require some explicit
proof as well.  In contrast, ``old-style'' Isabelle theories support batch
processing only, with the proof scripts collected in separate ML files.

The first actual command of any theory has to be $\THEORY$, starting a new
theory based on the merge of existing ones.  Just preceding $\THEORY$, there
may be an optional $\isarkeyword{header}$ declaration, which is relevant to
document preparation only; it acts very much like a special pre-theory markup
command (cf.\ \S\ref{sec:markup-thy} and \S\ref{sec:markup-thy}).  The theory
context may be also changed by $\CONTEXT$ without creating a new theory.  In
both cases, $\END$ concludes the theory development; it has to be the very
last command in a theory file.

\begin{rail}
  'header' text
  ;
  'theory' name '=' (name + '+') filespecs? ':'
  ;
  'context' name
  ;
  'end'
  ;;

  filespecs: 'files' ((name | parname) +);
\end{rail}

\begin{descr}
\item [$\isarkeyword{header}~text$] provides plain text markup just preceding
  the formal begin of a theory.  In actual document preparation the
  corresponding {\LaTeX} macro \verb,\isamarkupheader, may be redefined to
  produce chapter or section headings.  See also \S\ref{sec:markup-thy} and
  \S\ref{sec:markup-prf} for further markup commands.
  
\item [$\THEORY~A = B@1 + \cdots + B@n\colon$] commences a new theory $A$
  based on existing ones $B@1 + \cdots + B@n$.  Isabelle's theory loader
  system ensures that any of the base theories are properly loaded (and fully
  up-to-date when $\THEORY$ is executed interactively).  The optional
  $\isarkeyword{files}$ specification declares additional dependencies on ML
  files.  Unless put in parentheses, any file will be loaded immediately via
  $\isarcmd{use}$ (see also \S\ref{sec:ML}).  The optional ML file
  \texttt{$A$.ML} that may be associated with any theory should \emph{not} be
  included in $\isarkeyword{files}$, though.
  
\item [$\CONTEXT~B$] enters an existing theory context, basically in read-only
  mode, so only a limited set of commands may be performed without destroying
  the theory.  Just as for $\THEORY$, the theory loader ensures that $B$ is
  loaded and up-to-date.
  
\item [$\END$] concludes the current theory definition or context switch.
Note that this command cannot be undone, but the whole theory definition has
to be retracted.
\end{descr}


\subsection{Theory markup commands}\label{sec:markup-thy}

\indexisarcmd{chapter}\indexisarcmd{section}\indexisarcmd{subsection}
\indexisarcmd{subsubsection}\indexisarcmd{text}\indexisarcmd{text-raw}
\begin{matharray}{rcl}
  \isarcmd{chapter} & : & \isartrans{theory}{theory} \\
  \isarcmd{section} & : & \isartrans{theory}{theory} \\
  \isarcmd{subsection} & : & \isartrans{theory}{theory} \\
  \isarcmd{subsubsection} & : & \isartrans{theory}{theory} \\
  \isarcmd{text} & : & \isartrans{theory}{theory} \\
  \isarcmd{text_raw} & : & \isartrans{theory}{theory} \\
\end{matharray}

Apart from formal comments (see \S\ref{sec:comments}), markup commands provide
a structured way to insert text into the document generated from a theory (see
\cite{isabelle-sys} for more information on Isabelle's document preparation
tools).

\railalias{textraw}{text\_raw}
\railterm{textraw}

\begin{rail}
  ('chapter' | 'section' | 'subsection' | 'subsubsection' | 'text' | textraw) text
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{chapter}$, $\isarkeyword{section}$,
  $\isarkeyword{subsection}$, and $\isarkeyword{subsubsection}$] mark chapter
  and section headings.
\item [$\TEXT$] specifies paragraphs of plain text, including references to
  formal entities.\footnote{The latter feature is not yet supported.
    Nevertheless, any source text of the form
    ``\texttt{\at\ttlbrace$\dots$\ttrbrace}'' should be considered as reserved
    for future use.}
\item [$\isarkeyword{text_raw}$] inserts {\LaTeX} source into the output,
  without additional markup.  Thus the full range of document manipulations
  becomes available.  A typical application would be to emit
  \verb,\begin{comment}, and \verb,\end{comment}, commands to exclude certain
  parts from the final document.\footnote{This requires the \texttt{comment}
    package to be included in {\LaTeX}.}
\end{descr}

Any markup command (except $\isarkeyword{text_raw}$) corresponds to a {\LaTeX}
macro with the name prefixed by \verb,\isamarkup, (e.g.\ 
\verb,\isamarkupchapter, for $\isarkeyword{chapter}$). The \railqtoken{text}
argument is passed to that macro unchanged, i.e.\ further {\LaTeX} commands
may be included here as well.

\medskip Additional markup commands are available for proofs (see
\S\ref{sec:markup-prf}).  Also note that the $\isarkeyword{header}$
declaration (see \S\ref{sec:begin-thy}) admits to insert document markup
elements just preceding the actual theory definition.


\subsection{Type classes and sorts}\label{sec:classes}

\indexisarcmd{classes}\indexisarcmd{classrel}\indexisarcmd{defaultsort}
\begin{matharray}{rcl}
  \isarcmd{classes} & : & \isartrans{theory}{theory} \\
  \isarcmd{classrel} & : & \isartrans{theory}{theory} \\
  \isarcmd{defaultsort} & : & \isartrans{theory}{theory} \\
\end{matharray}

\begin{rail}
  'classes' (classdecl comment? +)
  ;
  'classrel' nameref '<' nameref comment?
  ;
  'defaultsort' sort comment?
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{classes}~c<\vec c$] declares class $c$ to be a subclass
  of existing classes $\vec c$.  Cyclic class structures are ruled out.
\item [$\isarkeyword{classrel}~c@1<c@2$] states a subclass relation between
  existing classes $c@1$ and $c@2$.  This is done axiomatically!  The
  $\isarkeyword{instance}$ command (see \S\ref{sec:axclass}) provides a way to
  introduce proven class relations.
\item [$\isarkeyword{defaultsort}~s$] makes sort $s$ the new default sort for
  any type variables given without sort constraints.  Usually, the default
  sort would be only changed when defining new logics.
\end{descr}


\subsection{Primitive types and type abbreviations}\label{sec:types-pure}

\indexisarcmd{typedecl}\indexisarcmd{types}\indexisarcmd{nonterminals}\indexisarcmd{arities}
\begin{matharray}{rcl}
  \isarcmd{types} & : & \isartrans{theory}{theory} \\
  \isarcmd{typedecl} & : & \isartrans{theory}{theory} \\
  \isarcmd{nonterminals} & : & \isartrans{theory}{theory} \\
  \isarcmd{arities} & : & \isartrans{theory}{theory} \\
\end{matharray}

\begin{rail}
  'types' (typespec '=' type infix? comment? +)
  ;
  'typedecl' typespec infix? comment?
  ;
  'nonterminals' (name +) comment?
  ;
  'arities' (nameref '::' arity comment? +)
  ;
\end{rail}

\begin{descr}
\item [$\TYPES~(\vec\alpha)t = \tau$] introduces \emph{type synonym}
  $(\vec\alpha)t$ for existing type $\tau$.  Unlike actual type definitions,
  as are available in Isabelle/HOL for example, type synonyms are just purely
  syntactic abbreviations without any logical significance.  Internally, type
  synonyms are fully expanded.
\item [$\isarkeyword{typedecl}~(\vec\alpha)t$] declares a new type constructor
  $t$, intended as an actual logical type.  Note that object-logics such as
  Isabelle/HOL override $\isarkeyword{typedecl}$ by their own version.
\item [$\isarkeyword{nonterminals}~\vec c$] declares $0$-ary type constructors
  $\vec c$ to act as purely syntactic types, i.e.\ nonterminal symbols of
  Isabelle's inner syntax of terms or types.
\item [$\isarkeyword{arities}~t::(\vec s)s$] augments Isabelle's order-sorted
  signature of types by new type constructor arities.  This is done
  axiomatically!  The $\isarkeyword{instance}$ command (see
  \S\ref{sec:axclass}) provides a way to introduce proven type arities.
\end{descr}


\subsection{Constants and simple definitions}\label{sec:consts}

\indexisarcmd{consts}\indexisarcmd{defs}\indexisarcmd{constdefs}\indexoutertoken{constdecl}
\begin{matharray}{rcl}
  \isarcmd{consts} & : & \isartrans{theory}{theory} \\
  \isarcmd{defs} & : & \isartrans{theory}{theory} \\
  \isarcmd{constdefs} & : & \isartrans{theory}{theory} \\
\end{matharray}

\begin{rail}
  'consts' (constdecl +)
  ;
  'defs' (axmdecl prop comment? +)
  ;
  'constdefs' (constdecl prop comment? +)
  ;

  constdecl: name '::' type mixfix? comment?
  ;
\end{rail}

\begin{descr}
\item [$\CONSTS~c::\sigma$] declares constant $c$ to have any instance of type
  scheme $\sigma$.  The optional mixfix annotations may attach concrete syntax
  to the constants declared.
\item [$\DEFS~name: eqn$] introduces $eqn$ as a definitional axiom for some
  existing constant.  See \cite[\S6]{isabelle-ref} for more details on the
  form of equations admitted as constant definitions.
\item [$\isarkeyword{constdefs}~c::\sigma~eqn$] combines declarations and
  definitions of constants, using canonical name $c_def$ for the definitional
  axiom.
\end{descr}


\subsection{Syntax and translations}\label{sec:syn-trans}

\indexisarcmd{syntax}\indexisarcmd{translations}
\begin{matharray}{rcl}
  \isarcmd{syntax} & : & \isartrans{theory}{theory} \\
  \isarcmd{translations} & : & \isartrans{theory}{theory} \\
\end{matharray}

\begin{rail}
  'syntax' ('(' name 'output'? ')')? (constdecl +)
  ;
  'translations' (transpat ('==' | '=>' | '<=') transpat comment? +)
  ;
  transpat: ('(' nameref ')')? string
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{syntax}~(mode)~decls$] is similar to $\CONSTS~decls$,
  except that the actual logical signature extension is omitted.  Thus the
  context free grammar of Isabelle's inner syntax may be augmented in
  arbitrary ways, independently of the logic.  The $mode$ argument refers to
  the print mode that the grammar rules belong; unless there is the
  \texttt{output} flag given, all productions are added both to the input and
  output grammar.
\item [$\isarkeyword{translations}~rules$] specifies syntactic translation
  rules (i.e.\ \emph{macros}): parse~/ print rules (\texttt{==}), parse rules
  (\texttt{=>}), or print rules (\texttt{<=}).  Translation patterns may be
  prefixed by the syntactic category to be used for parsing; the default is
  \texttt{logic}.
\end{descr}


\subsection{Axioms and theorems}

\indexisarcmd{axioms}\indexisarcmd{theorems}\indexisarcmd{lemmas}
\begin{matharray}{rcl}
  \isarcmd{axioms} & : & \isartrans{theory}{theory} \\
  \isarcmd{theorems} & : & \isartrans{theory}{theory} \\
  \isarcmd{lemmas} & : & \isartrans{theory}{theory} \\
\end{matharray}

\begin{rail}
  'axioms' (axmdecl prop comment? +)
  ;
  ('theorems' | 'lemmas') thmdef? thmrefs
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{axioms}~a: \phi$] introduces arbitrary statements as
  axioms of the meta-logic.  In fact, axioms are ``axiomatic theorems'', and
  may be referred later just as any other theorem.
  
  Axioms are usually only introduced when declaring new logical systems.
  Everyday work is typically done the hard way, with proper definitions and
  actual theorems.
\item [$\isarkeyword{theorems}~a = \vec b$] stores lists of existing theorems.
  Typical applications would also involve attributes, to augment the
  Simplifier context, for example.
\item [$\isarkeyword{lemmas}$] is similar to $\isarkeyword{theorems}$, but
  tags the results as ``lemma''.
\end{descr}


\subsection{Name spaces}

\indexisarcmd{global}\indexisarcmd{local}
\begin{matharray}{rcl}
  \isarcmd{global} & : & \isartrans{theory}{theory} \\
  \isarcmd{local} & : & \isartrans{theory}{theory} \\
\end{matharray}

Isabelle organizes any kind of name declarations (of types, constants,
theorems etc.)  by hierarchically structured name spaces.  Normally the user
never has to control the behavior of name space entry by hand, yet the
following commands provide some way to do so.

\begin{descr}
\item [$\isarkeyword{global}$ and $\isarkeyword{local}$] change the current
  name declaration mode.  Initially, theories start in $\isarkeyword{local}$
  mode, causing all names to be automatically qualified by the theory name.
  Changing this to $\isarkeyword{global}$ causes all names to be declared
  without the theory prefix, until $\isarkeyword{local}$ is declared again.
\end{descr}


\subsection{Incorporating ML code}\label{sec:ML}

\indexisarcmd{use}\indexisarcmd{ML}\indexisarcmd{ML-setup}\indexisarcmd{setup}
\begin{matharray}{rcl}
  \isarcmd{use} & : & \isartrans{\cdot}{\cdot} \\
  \isarcmd{ML} & : & \isartrans{\cdot}{\cdot} \\
  \isarcmd{ML_setup} & : & \isartrans{theory}{theory} \\
  \isarcmd{setup} & : & \isartrans{theory}{theory} \\
\end{matharray}

\railalias{MLsetup}{ML\_setup}
\railterm{MLsetup}

\begin{rail}
  'use' name
  ;
  ('ML' | MLsetup | 'setup') text
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{use}~file$] reads and executes ML commands from $file$.
  The current theory context (if present) is passed down to the ML session,
  but may not be modified.  Furthermore, the file name is checked with the
  $\isarkeyword{files}$ dependency declaration given in the theory header (see
  also \S\ref{sec:begin-thy}).
  
\item [$\isarkeyword{ML}~text$] executes ML commands from $text$.  The theory
  context is passed in the same way as for $\isarkeyword{use}$.
  
\item [$\isarkeyword{ML_setup}~text$] executes ML commands from $text$.  The
  theory context is passed down to the ML session, and fetched back
  afterwards.  Thus $text$ may actually change the theory as a side effect.
  
\item [$\isarkeyword{setup}~text$] changes the current theory context by
  applying $text$, which refers to an ML expression of type $(theory \to
  theory)~list$.  The $\isarkeyword{setup}$ command is the canonical way to
  initialize object-logic specific tools and packages written in ML.
\end{descr}


%FIXME remove!?
%\subsection{Syntax translation functions}

%\indexisarcmd{parse-ast-translation}\indexisarcmd{parse-translation}
%\indexisarcmd{print-translation}\indexisarcmd{typed-print-translation}
%\indexisarcmd{print-ast-translation}\indexisarcmd{token-translation}
%\begin{matharray}{rcl}
%  \isarcmd{parse_ast_translation} & : & \isartrans{theory}{theory} \\
%  \isarcmd{parse_translation} & : & \isartrans{theory}{theory} \\
%  \isarcmd{print_translation} & : & \isartrans{theory}{theory} \\
%  \isarcmd{typed_print_translation} & : & \isartrans{theory}{theory} \\
%  \isarcmd{print_ast_translation} & : & \isartrans{theory}{theory} \\
%  \isarcmd{token_translation} & : & \isartrans{theory}{theory} \\
%\end{matharray}

%Syntax translation functions written in ML admit almost arbitrary
%manipulations of Isabelle's inner syntax.  Any of the above commands have a
%single \railqtoken{text} argument that refers to an ML expression of
%appropriate type.  See \cite[\S8]{isabelle-ref} for more information on syntax
%transformations.


\subsection{Oracles}

\indexisarcmd{oracle}
\begin{matharray}{rcl}
  \isarcmd{oracle} & : & \isartrans{theory}{theory} \\
\end{matharray}

Oracles provide an interface to external reasoning systems, without giving up
control completely --- each theorem carries a derivation object recording any
oracle invocation.  See \cite[\S6]{isabelle-ref} for more information.

\begin{rail}
  'oracle' name '=' text comment?
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{oracle}~name=text$] declares oracle $name$ to be ML
  function $text$, which has to be of type $Sign\mathord.sg \times
  Object\mathord.T \to term$.
\end{descr}


\section{Proof commands}

Proof commands perform transitions of Isar/VM machine configurations, which
are block-structured, consisting of a stack of nodes with three main
components: logical proof context, current facts, and open goals.  Isar/VM
transitions are \emph{typed} according to the following three three different
modes of operation:
\begin{descr}
\item [$proof(prove)$] means that a new goal has just been stated that is now
  to be \emph{proven}; the next command may refine it by some proof method
  (read: tactic), and enter a sub-proof to establish the actual result.
\item [$proof(state)$] is like an internal theory mode: the context may be
  augmented by \emph{stating} additional assumptions, intermediate results
  etc.
\item [$proof(chain)$] is intermediate between $proof(state)$ and
  $proof(prove)$: existing facts (i.e.\ the contents of the special ``$this$''
  register) have been just picked up in order to be used when refining the
  goal claimed next.
\end{descr}


\subsection{Proof markup commands}\label{sec:markup-prf}

\indexisarcmd{sect}\indexisarcmd{subsect}\indexisarcmd{subsubsect}
\indexisarcmd{txt}\indexisarcmd{txt-raw}
\begin{matharray}{rcl}
  \isarcmd{sect} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{subsect} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{subsubsect} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{txt} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{txt_raw} & : & \isartrans{proof(state)}{proof(state)} \\
\end{matharray}

These markup commands for proof mode closely correspond to the ones of theory
mode (see \S\ref{sec:markup-thy}).  Note that $\isarkeyword{txt_raw}$ is
special in the same way as $\isarkeyword{text_raw}$.

\railalias{txtraw}{txt\_raw}
\railterm{txtraw}

\begin{rail}
  ('sect' | 'subsect' | 'subsubsect' | 'txt' | txtraw) text
  ;
\end{rail}


\subsection{Proof context}\label{sec:proof-context}

\indexisarcmd{fix}\indexisarcmd{assume}\indexisarcmd{presume}\indexisarcmd{def}
\begin{matharray}{rcl}
  \isarcmd{fix} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{assume} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{presume} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{def} & : & \isartrans{proof(state)}{proof(state)} \\
\end{matharray}

The logical proof context consists of fixed variables and assumptions.  The
former closely correspond to Skolem constants, or meta-level universal
quantification as provided by the Isabelle/Pure logical framework.
Introducing some \emph{arbitrary, but fixed} variable via $\FIX x$ results in
a local value that may be used in the subsequent proof as any other variable
or constant.  Furthermore, any result $\edrv \phi[x]$ exported from the
context will be universally closed wrt.\ $x$ at the outermost level: $\edrv
\All x \phi$ (this is expressed using Isabelle's meta-variables).

Similarly, introducing some assumption $\chi$ has two effects.  On the one
hand, a local theorem is created that may be used as a fact in subsequent
proof steps.  On the other hand, any result $\chi \drv \phi$ exported from the
context becomes conditional wrt.\ the assumption: $\edrv \chi \Imp \phi$.
Thus, solving an enclosing goal using such a result would basically introduce
a new subgoal stemming from the assumption.  How this situation is handled
depends on the actual version of assumption command used: while $\ASSUMENAME$
insists on solving the subgoal by unification with some premise of the goal,
$\PRESUMENAME$ leaves the subgoal unchanged in order to be proved later by the
user.

Local definitions, introduced by $\DEF{}{x \equiv t}$, are achieved by
combining $\FIX x$ with another version of assumption that causes any
hypothetical equation $x \equiv t$ to be eliminated by the reflexivity rule.
Thus, exporting some result $x \equiv t \drv \phi[x]$ yields $\edrv \phi[t]$.

\begin{rail}
  'fix' (vars + 'and') comment?
  ;
  ('assume' | 'presume') (assm comment? + 'and')
  ;
  'def' thmdecl? \\ var '==' term termpat? comment?
  ;

  var: name ('::' type)?
  ;
  vars: (name+) ('::' type)?
  ;
  assm: thmdecl? (prop proppat? +)
  ;
\end{rail}

\begin{descr}
\item [$\FIX{x}$] introduces a local \emph{arbitrary, but fixed} variable $x$.
\item [$\ASSUME{a}{\Phi}$ and $\PRESUME{a}{\Phi}$] introduce local theorems
  $\Phi$ by assumption.  Subsequent results applied to an enclosing goal
  (e.g.\ by $\SHOWNAME$) are handled as follows: $\ASSUMENAME$ expects to be
  able to unify with existing premises in the goal, while $\PRESUMENAME$
  leaves $\Phi$ as new subgoals.
  
  Several lists of assumptions may be given (separated by
  $\isarkeyword{and}$); the resulting list of current facts consists of all of
  these concatenated.
\item [$\DEF{a}{x \equiv t}$] introduces a local (non-polymorphic) definition.
  In results exported from the context, $x$ is replaced by $t$.  Basically,
  $\DEF{}{x \equiv t}$ abbreviates $\FIX{x}~\ASSUME{}{x \equiv t}$, with the
  resulting hypothetical equation solved by reflexivity.
  
  The default name for the definitional equation is $x_def$.
\end{descr}

The special name $prems$\indexisarthm{prems} refers to all assumptions of the
current context as a list of theorems.


\subsection{Facts and forward chaining}

\indexisarcmd{note}\indexisarcmd{then}\indexisarcmd{from}\indexisarcmd{with}
\begin{matharray}{rcl}
  \isarcmd{note} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarcmd{then} & : & \isartrans{proof(state)}{proof(chain)} \\
  \isarcmd{from} & : & \isartrans{proof(state)}{proof(chain)} \\
  \isarcmd{with} & : & \isartrans{proof(state)}{proof(chain)} \\
\end{matharray}

New facts are established either by assumption or proof of local statements.
Any fact will usually be involved in further proofs, either as explicit
arguments of proof methods or when forward chaining towards the next goal via
$\THEN$ (and variants).  Note that the special theorem name
$this$\indexisarthm{this} refers to the most recently established facts.
\begin{rail}
  'note' thmdef? thmrefs comment?
  ;
  'then' comment?
  ;
  ('from' | 'with') thmrefs comment?
  ;
\end{rail}

\begin{descr}
\item [$\NOTE{a}{\vec b}$] recalls existing facts $\vec b$, binding the result
  as $a$.  Note that attributes may be involved as well, both on the left and
  right hand sides.
\item [$\THEN$] indicates forward chaining by the current facts in order to
  establish the goal to be claimed next.  The initial proof method invoked to
  refine that will be offered the facts to do ``anything appropriate'' (cf.\ 
  also \S\ref{sec:proof-steps}).  For example, method $rule$ (see
  \S\ref{sec:pure-meth}) would typically do an elimination rather than an
  introduction.  Automatic methods usually insert the facts into the goal
  state before operation.
\item [$\FROM{\vec b}$] abbreviates $\NOTE{}{\vec b}~\THEN$; thus $\THEN$ is
  equivalent to $\FROM{this}$.
\item [$\WITH{\vec b}$] abbreviates $\FROM{\vec b~facts}$; thus the forward
  chaining is from earlier facts together with the current ones.
\end{descr}

Basic proof methods (such as $rule$, see \S\ref{sec:pure-meth}) expect
multiple facts to be given in their proper order, corresponding to a prefix of
the premises of the rule involved.  Note that positions may be easily skipped
using a form like $\FROM{\text{\texttt{_}}~a~b}$, for example.  This involves
the trivial rule $\PROP\psi \Imp \PROP\psi$, which is bound in Isabelle/Pure
as ``\texttt{_}'' (underscore).\indexisarthm{_@\texttt{_}}


\subsection{Goal statements}

\indexisarcmd{theorem}\indexisarcmd{lemma}
\indexisarcmd{have}\indexisarcmd{show}\indexisarcmd{hence}\indexisarcmd{thus}
\begin{matharray}{rcl}
  \isarcmd{theorem} & : & \isartrans{theory}{proof(prove)} \\
  \isarcmd{lemma} & : & \isartrans{theory}{proof(prove)} \\
  \isarcmd{have} & : & \isartrans{proof(state) ~|~ proof(chain)}{proof(prove)} \\
  \isarcmd{show} & : & \isartrans{proof(state) ~|~ proof(chain)}{proof(prove)} \\
  \isarcmd{hence} & : & \isartrans{proof(state)}{proof(prove)} \\
  \isarcmd{thus} & : & \isartrans{proof(state)}{proof(prove)} \\
\end{matharray}

Proof mode is entered from theory mode by initial goal commands $\THEOREMNAME$
and $\LEMMANAME$.  New local goals may be claimed within proof mode as well.
Four variants are available, indicating whether the result is meant to solve
some pending goal or whether forward chaining is employed.

\begin{rail}
  ('theorem' | 'lemma') goal
  ;
  ('have' | 'show' | 'hence' | 'thus') goal
  ;

  goal: thmdecl? proppat comment?
  ;
\end{rail}

\begin{descr}
\item [$\THEOREM{a}{\phi}$] enters proof mode with $\phi$ as main goal,
  eventually resulting in some theorem $\turn \phi$ put back into the theory.
\item [$\LEMMA{a}{\phi}$] is similar to $\THEOREMNAME$, but tags the result as
  ``lemma''.
\item [$\HAVE{a}{\phi}$] claims a local goal, eventually resulting in a
  theorem with the current assumption context as hypotheses.
\item [$\SHOW{a}{\phi}$] is similar to $\HAVE{a}{\phi}$, but solves some
  pending goal with the result \emph{exported} into the corresponding context
  (cf.\ \S\ref{sec:proof-context}).
\item [$\HENCENAME$] abbreviates $\THEN~\HAVENAME$, i.e.\ claims a local goal
  to be proven by forward chaining the current facts.  Note that $\HENCENAME$
  is also equivalent to $\FROM{this}~\HAVENAME$.
\item [$\THUSNAME$] abbreviates $\THEN~\SHOWNAME$.  Note that $\THUSNAME$ is
  also equivalent to $\FROM{this}~\SHOWNAME$.
\end{descr}


\subsection{Initial and terminal proof steps}\label{sec:proof-steps}

\indexisarcmd{proof}\indexisarcmd{qed}\indexisarcmd{by}
\indexisarcmd{.}\indexisarcmd{..}\indexisarcmd{sorry}
\begin{matharray}{rcl}
  \isarcmd{proof} & : & \isartrans{proof(prove)}{proof(state)} \\
  \isarcmd{qed} & : & \isartrans{proof(state)}{proof(state) ~|~ theory} \\
  \isarcmd{by} & : & \isartrans{proof(prove)}{proof(state) ~|~ theory} \\
  \isarcmd{.\,.} & : & \isartrans{proof(prove)}{proof(state) ~|~ theory} \\
  \isarcmd{.} & : & \isartrans{proof(prove)}{proof(state) ~|~ theory} \\
  \isarcmd{sorry} & : & \isartrans{proof(prove)}{proof(state) ~|~ theory} \\
\end{matharray}

Arbitrary goal refinement via tactics is considered harmful.  Consequently the
Isar framework admits proof methods to be invoked in two places only.
\begin{enumerate}
\item An \emph{initial} refinement step $\PROOF{m@1}$ reduces a newly stated
  goal to a number of sub-goals that are to be solved later.  Facts are passed
  to $m@1$ for forward chaining, if so indicated by $proof(chain)$ mode.
  
\item A \emph{terminal} conclusion step $\QED{m@2}$ is intended to solve
  remaining goals.  No facts are passed to $m@2$.
\end{enumerate}

The only other proper way to affect pending goals is by $\SHOWNAME$ (or
$\THUSNAME$), which involves an explicit statement of what is to be solved.

\medskip

Also note that initial proof methods should either solve the goal completely,
or constitute some well-understood reduction to new sub-goals.  Arbitrary
automatic proof tools that are prone leave a large number of badly structured
sub-goals are no help in continuing the proof document in any intelligible
way.
%FIXME
%A more appropriate technique would be to $\SHOWNAME$ some non-trivial
%reduction as an explicit rule, which is solved completely by some automated
%method, and then applied to some pending goal.

\medskip

Unless given explicitly by the user, the default initial method is
``$default$'', which is usually set up to apply a single standard elimination
or introduction rule according to the topmost symbol involved.  There is no
separate default terminal method.  In any case, any goals left after that are
solved by assumption as the very last step.

\begin{rail}
  'proof' interest? meth? comment?
  ;
  'qed' meth? comment?
  ;
  'by' meth meth? comment?
  ;
  ('.' | '..' | 'sorry') comment?
  ;

  meth: method interest?
  ;
\end{rail}

\begin{descr}
\item [$\PROOF{m@1}$] refines the goal by proof method $m@1$; facts for
  forward chaining are passed if so indicated by $proof(chain)$ mode.
\item [$\QED{m@2}$] refines any remaining goals by proof method $m@2$ and
  concludes the sub-proof by assumption.  If the goal had been $\SHOWNAME$ (or
  $\THUSNAME$), some pending sub-goal is solved as well by the rule resulting
  from the result \emph{exported} into the enclosing goal context.  Thus
  $\QEDNAME$ may fail for two reasons: either $m@2$ fails, or the resulting
  rule does not fit to any pending goal\footnote{This includes any additional
    ``strong'' assumptions as introduced by $\ASSUMENAME$.} of the enclosing
  context.  Debugging such a situation might involve temporarily changing
  $\SHOWNAME$ into $\HAVENAME$, or weakening the local context by replacing
  some occurrences of $\ASSUMENAME$ by $\PRESUMENAME$.
\item [$\BYY{m@1}{m@2}$] is a \emph{terminal proof}\index{proof!terminal}; it
  abbreviates $\PROOF{m@1}~\QED{m@2}$, with backtracking across both methods,
  though.  Debugging an unsuccessful $\BYY{m@1}{m@2}$ commands might be done
  by expanding its definition; in many cases $\PROOF{m@1}$ is already
  sufficient to see what is going wrong.
\item [``$\DDOT$''] is a \emph{default proof}\index{proof!default}; it
  abbreviates $\BY{default}$.
\item [``$\DOT$''] is a \emph{trivial proof}\index{proof!trivial}; it
  abbreviates $\BY{assumption}$.
\item [$\isarkeyword{sorry}$] is a \emph{fake proof}\index{proof!fake};
  provided that the \texttt{quick_and_dirty} flag is enabled,
  $\isarkeyword{sorry}$ pretends to solve the goal without further ado.  Of
  course, the result is a fake theorem only, involving some oracle in its
  internal derivation object (this is indicated as ``$[!]$'' in the printed
  result).  The main application of $\isarkeyword{sorry}$ is to support
  experimentation and top-down proof development.
\end{descr}


\subsection{Improper proof steps}

The following commands emulate unstructured tactic scripts to some extent.
While these are anathema for writing proper Isar proof documents, they might
come in handy for interactive exploration and debugging.

\indexisarcmd{apply}\indexisarcmd{then-apply}\indexisarcmd{back}
\begin{matharray}{rcl}
  \isarcmd{apply}^* & : & \isartrans{proof}{proof} \\
  \isarcmd{then_apply}^* & : & \isartrans{proof}{proof} \\
  \isarcmd{back}^* & : & \isartrans{proof}{proof} \\
\end{matharray}

\railalias{thenapply}{then\_apply}
\railterm{thenapply}

\begin{rail}
  'apply' method
  ;
  thenapply method
  ;
  'back'
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{apply}~(m)$] applies proof method $m$ in the plain old
  tactic sense.  Facts for forward chaining are reset.
\item [$\isarkeyword{then_apply}~(m)$] is similar to $\isarkeyword{apply}$,
  but keeps the goal's facts.
\item [$\isarkeyword{back}$] does back-tracking over the result sequence of
  the latest proof command.\footnote{Unlike the ML function \texttt{back}
    \cite{isabelle-ref}, the Isar command does not search upwards for further
    branch points.} Basically, any proof command may return multiple results.
\end{descr}


\subsection{Term abbreviations}\label{sec:term-abbrev}

\indexisarcmd{let}
\begin{matharray}{rcl}
  \isarcmd{let} & : & \isartrans{proof(state)}{proof(state)} \\
  \isarkeyword{is} & : & syntax \\
\end{matharray}

Abbreviations may be either bound by explicit $\LET{p \equiv t}$ statements,
or by annotating assumptions or goal statements with a list of patterns
$\ISS{p@1\;\dots}{p@n}$.  In both cases, higher-order matching is invoked to
bind extra-logical term variables, which may be either named schematic
variables of the form $\Var{x}$, or nameless dummies ``\texttt{_}''
(underscore).\indexisarvar{_@\texttt{_}} Note that in the $\LETNAME$ form the
patterns occur on the left-hand side, while the $\ISNAME$ patterns are in
postfix position.

Term abbreviations are quite different from actual local definitions as
introduced via $\DEFNAME$ (see \S\ref{sec:proof-context}).  The latter are
visible within the logic as actual equations, while abbreviations disappear
during the input process just after type checking.

\begin{rail}
  'let' ((term + 'as') '=' term comment? + 'and')
  ;  
\end{rail}

The syntax of $\ISNAME$ patterns follows \railnonterm{termpat} or
\railnonterm{proppat} (see \S\ref{sec:term-pats}).

\begin{descr}
\item [$\LET{\vec p = \vec t}$] binds any text variables in patters $\vec p$
  by simultaneous higher-order matching against terms $\vec t$.
\item [$\IS{\vec p}$] resembles $\LETNAME$, but matches $\vec p$ against the
  preceding statement.  Also note that $\ISNAME$ is not a separate command,
  but part of others (such as $\ASSUMENAME$, $\HAVENAME$ etc.).
\end{descr}

A few \emph{automatic} term abbreviations\index{term abbreviations} for goals
and facts are available as well.  For any open goal,
$\Var{thesis_prop}$\indexisarvar{thesis-prop} refers to the full proposition
(which may be a rule), $\Var{thesis_concl}$\indexisarvar{thesis-concl} to its
(atomic) conclusion, and $\Var{thesis}$\indexisarvar{thesis} to its
object-logical statement.  The latter two abstract over any meta-level
parameters.

Fact statements resulting from assumptions or finished goals are bound as
$\Var{this_prop}$\indexisarvar{this-prop},
$\Var{this_concl}$\indexisarvar{this-concl}, and
$\Var{this}$\indexisarvar{this}, similar to $\Var{thesis}$ above.  In case
$\Var{this}$ refers to an object-logic statement that is an application
$f(t)$, then $t$ is bound to the special text variable
``$\dots$''\indexisarvar{\dots} (three dots).  The canonical application of
the latter are calculational proofs (see \S\ref{sec:calculation}).


\subsection{Block structure}

\indexisarcmd{next}\indexisarcmd{\{\{}\indexisarcmd{\}\}}
\begin{matharray}{rcl}
  \isarcmd{next} & : & \isartrans{proof(state)}{proof(state)} \\
  \BG & : & \isartrans{proof(state)}{proof(state)} \\
  \EN & : & \isartrans{proof(state)}{proof(state)} \\
\end{matharray}

While Isar is inherently block-structured, opening and closing blocks is
mostly handled rather casually, with little explicit user-intervention.  Any
local goal statement automatically opens \emph{two} blocks, which are closed
again when concluding the sub-proof (by $\QEDNAME$ etc.).  Sections of
different context within a sub-proof may be switched via $\isarkeyword{next}$,
which is just a single block-close followed by block-open again.  Thus the
effect of $\isarkeyword{next}$ to reset the local proof context. There is no
goal focus involved here!

For slightly more advanced applications, there are explicit block parentheses
as well.  These typically achieve a stronger forward style of reasoning.

\begin{descr}
\item [$\isarkeyword{next}$] switches to a fresh block within a sub-proof,
  resetting the local context to the initial one.
\item [$\isarkeyword{\{\{}$ and $\isarkeyword{\}\}}$] explicitly open and
  close blocks.  Any current facts pass through ``$\isarkeyword{\{\{}$''
  unchanged, while ``$\isarkeyword{\}\}}$'' causes any result to be
  \emph{exported} into the enclosing context.  Thus fixed variables are
  generalized, assumptions discharged, and local definitions unfolded (cf.\ 
  \S\ref{sec:proof-context}).  There is no difference of $\ASSUMENAME$ and
  $\PRESUMENAME$ in this mode of forward reasoning --- in contrast to plain
  backward reasoning with the result exported at $\SHOWNAME$ time.
\end{descr}


\section{Other commands}

\subsection{Diagnostics}

\indexisarcmd{thm}\indexisarcmd{term}\indexisarcmd{prop}\indexisarcmd{typ}
\begin{matharray}{rcl}
  \isarcmd{thm} & : & \isarkeep{theory~|~proof} \\
  \isarcmd{term} & : & \isarkeep{theory~|~proof} \\
  \isarcmd{prop} & : & \isarkeep{theory~|~proof} \\
  \isarcmd{typ} & : & \isarkeep{theory~|~proof} \\
\end{matharray}

These commands are not part of the actual Isabelle/Isar syntax, but assist
interactive development.  Also note that $undo$ does not apply here, since the
theory or proof configuration is not changed.

\begin{rail}
  'thm' thmrefs
  ;
  'term' term
  ;
  'prop' prop
  ;
  'typ' type
  ;
\end{rail}

\begin{descr}
\item [$\isarkeyword{thm}~thms$] retrieves lists of theorems from the current
  theory or proof context.  Note that any attributes included in the theorem
  specifications are applied to a temporary context derived from the current
  theory or proof; the result is discarded, i.e.\ attributes involved in
  $thms$ do not have any permanent effect.
\item [$\isarkeyword{term}~t$, $\isarkeyword{prop}~\phi$] read, type-check and
  print terms or propositions according to the current theory or proof
  context; the inferred type of $t$ is output as well.  Note that these
  commands are also useful in inspecting the current environment of term
  abbreviations.
\item [$\isarkeyword{typ}~\tau$] reads and prints types of the meta-logic
  according to the current theory or proof context.
\end{descr}


\subsection{System operations}

\indexisarcmd{cd}\indexisarcmd{pwd}\indexisarcmd{use-thy}\indexisarcmd{use-thy-only}
\indexisarcmd{update-thy}\indexisarcmd{update-thy-only}
\begin{matharray}{rcl}
  \isarcmd{cd} & : & \isarkeep{\cdot} \\
  \isarcmd{pwd} & : & \isarkeep{\cdot} \\
  \isarcmd{use_thy} & : & \isarkeep{\cdot} \\
  \isarcmd{use_thy_only} & : & \isarkeep{\cdot} \\
  \isarcmd{update_thy} & : & \isarkeep{\cdot} \\
  \isarcmd{update_thy_only} & : & \isarkeep{\cdot} \\
\end{matharray}

\begin{descr}
\item [$\isarkeyword{cd}~name$] changes the current directory of the Isabelle
  process.
\item [$\isarkeyword{pwd}~$] prints the current working directory.
\item [$\isarkeyword{use_thy}$, $\isarkeyword{use_thy_only}$,
  $\isarkeyword{update_thy}$, $\isarkeyword{update_thy_only}$] load some
  theory given as $name$ argument.  These commands are basically the same as
  the corresponding ML functions\footnote{The ML versions also change the
    implicit theory context to that of the theory loaded.}  (see also
  \cite[\S1,\S6]{isabelle-ref}).  Note that both the ML and Isar versions may
  load new- and old-style theories alike.
\end{descr}

These system commands are scarcely used when working with the Proof~General
interface, since loading of theories is done fully transparently.

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