doc-src/TutorialI/Advanced/document/simp.tex

more accurate ROOT.ML;

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\isamarkupsection{Simplification%
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\begin{isamarkuptext}%
\label{sec:simplification-II}\index{simplification|(}
This section describes features not covered until now.  It also
outlines the simplification process itself, which can be helpful
when the simplifier does not do what you expect of it.%
\end{isamarkuptext}%
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\isamarkupsubsection{Advanced Features%
}
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\isamarkupsubsubsection{Congruence Rules%
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\begin{isamarkuptext}%
\label{sec:simp-cong}
While simplifying the conclusion $Q$
of $P \Imp Q$, it is legal to use the assumption $P$.
For $\Imp$ this policy is hardwired, but 
contextual information can also be made available for other
operators. For example, \isa{xs\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{5B}{\isacharbrackleft}}{\isaliteral{5D}{\isacharbrackright}}\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ xs\ {\isaliteral{40}{\isacharat}}\ xs\ {\isaliteral{3D}{\isacharequal}}\ xs} simplifies to \isa{True} because we may use \isa{xs\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{5B}{\isacharbrackleft}}{\isaliteral{5D}{\isacharbrackright}}} when simplifying \isa{xs\ {\isaliteral{40}{\isacharat}}\ xs\ {\isaliteral{3D}{\isacharequal}}\ xs}. The generation of contextual information during simplification is
controlled by so-called \bfindex{congruence rules}. This is the one for
\isa{{\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}}:
\begin{isabelle}%
\ \ \ \ \ {\isaliteral{5C3C6C6272616B6B3E}{\isasymlbrakk}}P\ {\isaliteral{3D}{\isacharequal}}\ P{\isaliteral{27}{\isacharprime}}{\isaliteral{3B}{\isacharsemicolon}}\ P{\isaliteral{27}{\isacharprime}}\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ Q\ {\isaliteral{3D}{\isacharequal}}\ Q{\isaliteral{27}{\isacharprime}}{\isaliteral{5C3C726272616B6B3E}{\isasymrbrakk}}\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}P\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ Q{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}P{\isaliteral{27}{\isacharprime}}\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ Q{\isaliteral{27}{\isacharprime}}{\isaliteral{29}{\isacharparenright}}%
\end{isabelle}
It should be read as follows:
In order to simplify \isa{P\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ Q} to \isa{P{\isaliteral{27}{\isacharprime}}\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ Q{\isaliteral{27}{\isacharprime}}},
simplify \isa{P} to \isa{P{\isaliteral{27}{\isacharprime}}}
and assume \isa{P{\isaliteral{27}{\isacharprime}}} when simplifying \isa{Q} to \isa{Q{\isaliteral{27}{\isacharprime}}}.

Here are some more examples.  The congruence rules for bounded
quantifiers supply contextual information about the bound variable:
\begin{isabelle}%
\ \ \ \ \ {\isaliteral{5C3C6C6272616B6B3E}{\isasymlbrakk}}A\ {\isaliteral{3D}{\isacharequal}}\ B{\isaliteral{3B}{\isacharsemicolon}}\ {\isaliteral{5C3C416E643E}{\isasymAnd}}x{\isaliteral{2E}{\isachardot}}\ x\ {\isaliteral{5C3C696E3E}{\isasymin}}\ B\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ P\ x\ {\isaliteral{3D}{\isacharequal}}\ Q\ x{\isaliteral{5C3C726272616B6B3E}{\isasymrbrakk}}\isanewline
\isaindent{\ \ \ \ \ }{\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C666F72616C6C3E}{\isasymforall}}x{\isaliteral{5C3C696E3E}{\isasymin}}A{\isaliteral{2E}{\isachardot}}\ P\ x{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C666F72616C6C3E}{\isasymforall}}x{\isaliteral{5C3C696E3E}{\isasymin}}B{\isaliteral{2E}{\isachardot}}\ Q\ x{\isaliteral{29}{\isacharparenright}}%
\end{isabelle}
One congruence rule for conditional expressions supplies contextual
information for simplifying the \isa{then} and \isa{else} cases:
\begin{isabelle}%
\ \ \ \ \ {\isaliteral{5C3C6C6272616B6B3E}{\isasymlbrakk}}b\ {\isaliteral{3D}{\isacharequal}}\ c{\isaliteral{3B}{\isacharsemicolon}}\ c\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ x\ {\isaliteral{3D}{\isacharequal}}\ u{\isaliteral{3B}{\isacharsemicolon}}\ {\isaliteral{5C3C6E6F743E}{\isasymnot}}\ c\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ y\ {\isaliteral{3D}{\isacharequal}}\ v{\isaliteral{5C3C726272616B6B3E}{\isasymrbrakk}}\isanewline
\isaindent{\ \ \ \ \ }{\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}if\ b\ then\ x\ else\ y{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}if\ c\ then\ u\ else\ v{\isaliteral{29}{\isacharparenright}}%
\end{isabelle}
An alternative congruence rule for conditional expressions
actually \emph{prevents} simplification of some arguments:
\begin{isabelle}%
\ \ \ \ \ b\ {\isaliteral{3D}{\isacharequal}}\ c\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}if\ b\ then\ x\ else\ y{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}if\ c\ then\ x\ else\ y{\isaliteral{29}{\isacharparenright}}%
\end{isabelle}
Only the first argument is simplified; the others remain unchanged.
This makes simplification much faster and is faithful to the evaluation
strategy in programming languages, which is why this is the default
congruence rule for \isa{if}. Analogous rules control the evaluation of
\isa{case} expressions.

You can declare your own congruence rules with the attribute \attrdx{cong},
either globally, in the usual manner,
\begin{quote}
\isacommand{declare} \textit{theorem-name} \isa{{\isaliteral{5B}{\isacharbrackleft}}cong{\isaliteral{5D}{\isacharbrackright}}}
\end{quote}
or locally in a \isa{simp} call by adding the modifier
\begin{quote}
\isa{cong{\isaliteral{3A}{\isacharcolon}}} \textit{list of theorem names}
\end{quote}
The effect is reversed by \isa{cong\ del} instead of \isa{cong}.

\begin{warn}
The congruence rule \isa{conj{\isaliteral{5F}{\isacharunderscore}}cong}
\begin{isabelle}%
\ \ \ \ \ {\isaliteral{5C3C6C6272616B6B3E}{\isasymlbrakk}}P\ {\isaliteral{3D}{\isacharequal}}\ P{\isaliteral{27}{\isacharprime}}{\isaliteral{3B}{\isacharsemicolon}}\ P{\isaliteral{27}{\isacharprime}}\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ Q\ {\isaliteral{3D}{\isacharequal}}\ Q{\isaliteral{27}{\isacharprime}}{\isaliteral{5C3C726272616B6B3E}{\isasymrbrakk}}\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}P\ {\isaliteral{5C3C616E643E}{\isasymand}}\ Q{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}P{\isaliteral{27}{\isacharprime}}\ {\isaliteral{5C3C616E643E}{\isasymand}}\ Q{\isaliteral{27}{\isacharprime}}{\isaliteral{29}{\isacharparenright}}%
\end{isabelle}
\par\noindent
is occasionally useful but is not a default rule; you have to declare it explicitly.
\end{warn}%
\end{isamarkuptext}%
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\isamarkupsubsubsection{Permutative Rewrite Rules%
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\begin{isamarkuptext}%
\index{rewrite rules!permutative|bold}%
An equation is a \textbf{permutative rewrite rule} if the left-hand
side and right-hand side are the same up to renaming of variables.  The most
common permutative rule is commutativity: \isa{x\ {\isaliteral{2B}{\isacharplus}}\ y\ {\isaliteral{3D}{\isacharequal}}\ y\ {\isaliteral{2B}{\isacharplus}}\ x}.  Other examples
include \isa{x\ {\isaliteral{2D}{\isacharminus}}\ y\ {\isaliteral{2D}{\isacharminus}}\ z\ {\isaliteral{3D}{\isacharequal}}\ x\ {\isaliteral{2D}{\isacharminus}}\ z\ {\isaliteral{2D}{\isacharminus}}\ y} in arithmetic and \isa{insert\ x\ {\isaliteral{28}{\isacharparenleft}}insert\ y\ A{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ insert\ y\ {\isaliteral{28}{\isacharparenleft}}insert\ x\ A{\isaliteral{29}{\isacharparenright}}} for sets. Such rules are problematic because
once they apply, they can be used forever. The simplifier is aware of this
danger and treats permutative rules by means of a special strategy, called
\bfindex{ordered rewriting}: a permutative rewrite
rule is only applied if the term becomes smaller with respect to a fixed
lexicographic ordering on terms. For example, commutativity rewrites
\isa{b\ {\isaliteral{2B}{\isacharplus}}\ a} to \isa{a\ {\isaliteral{2B}{\isacharplus}}\ b}, but then stops because \isa{a\ {\isaliteral{2B}{\isacharplus}}\ b} is strictly
smaller than \isa{b\ {\isaliteral{2B}{\isacharplus}}\ a}.  Permutative rewrite rules can be turned into
simplification rules in the usual manner via the \isa{simp} attribute; the
simplifier recognizes their special status automatically.

Permutative rewrite rules are most effective in the case of
associative-commutative functions.  (Associativity by itself is not
permutative.)  When dealing with an AC-function~$f$, keep the
following points in mind:
\begin{itemize}\index{associative-commutative function}
  
\item The associative law must always be oriented from left to right,
  namely $f(f(x,y),z) = f(x,f(y,z))$.  The opposite orientation, if
  used with commutativity, can lead to nontermination.

\item To complete your set of rewrite rules, you must add not just
  associativity~(A) and commutativity~(C) but also a derived rule, {\bf
    left-com\-mut\-ativ\-ity} (LC): $f(x,f(y,z)) = f(y,f(x,z))$.
\end{itemize}
Ordered rewriting with the combination of A, C, and LC sorts a term
lexicographically:
\[\def\maps#1{~\stackrel{#1}{\leadsto}~}
 f(f(b,c),a) \maps{A} f(b,f(c,a)) \maps{C} f(b,f(a,c)) \maps{LC} f(a,f(b,c)) \]

Note that ordered rewriting for \isa{{\isaliteral{2B}{\isacharplus}}} and \isa{{\isaliteral{2A}{\isacharasterisk}}} on numbers is rarely
necessary because the built-in arithmetic prover often succeeds without
such tricks.%
\end{isamarkuptext}%
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\isamarkupsubsection{How the Simplifier Works%
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\begin{isamarkuptext}%
\label{sec:SimpHow}
Roughly speaking, the simplifier proceeds bottom-up: subterms are simplified
first.  A conditional equation is only applied if its condition can be
proved, again by simplification.  Below we explain some special features of
the rewriting process.%
\end{isamarkuptext}%
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\isamarkupsubsubsection{Higher-Order Patterns%
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\begin{isamarkuptext}%
\index{simplification rule|(}
So far we have pretended the simplifier can deal with arbitrary
rewrite rules. This is not quite true.  For reasons of feasibility,
the simplifier expects the
left-hand side of each rule to be a so-called \emph{higher-order
pattern}~\cite{nipkow-patterns}\indexbold{patterns!higher-order}. 
This restricts where
unknowns may occur.  Higher-order patterns are terms in $\beta$-normal
form.  (This means there are no subterms of the form $(\lambda x. M)(N)$.)  
Each occurrence of an unknown is of the form
$\Var{f}~x@1~\dots~x@n$, where the $x@i$ are distinct bound
variables. Thus all ordinary rewrite rules, where all unknowns are
of base type, for example \isa{{\isaliteral{3F}{\isacharquery}}a\ {\isaliteral{2B}{\isacharplus}}\ {\isaliteral{3F}{\isacharquery}}b\ {\isaliteral{2B}{\isacharplus}}\ {\isaliteral{3F}{\isacharquery}}c\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{3F}{\isacharquery}}a\ {\isaliteral{2B}{\isacharplus}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{3F}{\isacharquery}}b\ {\isaliteral{2B}{\isacharplus}}\ {\isaliteral{3F}{\isacharquery}}c{\isaliteral{29}{\isacharparenright}}}, are acceptable: if an unknown is
of base type, it cannot have any arguments. Additionally, the rule
\isa{{\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C666F72616C6C3E}{\isasymforall}}x{\isaliteral{2E}{\isachardot}}\ {\isaliteral{3F}{\isacharquery}}P\ x\ {\isaliteral{5C3C616E643E}{\isasymand}}\ {\isaliteral{3F}{\isacharquery}}Q\ x{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C666F72616C6C3E}{\isasymforall}}x{\isaliteral{2E}{\isachardot}}\ {\isaliteral{3F}{\isacharquery}}P\ x{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{5C3C616E643E}{\isasymand}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C666F72616C6C3E}{\isasymforall}}x{\isaliteral{2E}{\isachardot}}\ {\isaliteral{3F}{\isacharquery}}Q\ x{\isaliteral{29}{\isacharparenright}}{\isaliteral{29}{\isacharparenright}}} is also acceptable, in
both directions: all arguments of the unknowns \isa{{\isaliteral{3F}{\isacharquery}}P} and
\isa{{\isaliteral{3F}{\isacharquery}}Q} are distinct bound variables.

If the left-hand side is not a higher-order pattern, all is not lost.
The simplifier will still try to apply the rule provided it
matches directly: without much $\lambda$-calculus hocus
pocus.  For example, \isa{{\isaliteral{28}{\isacharparenleft}}{\isaliteral{3F}{\isacharquery}}f\ {\isaliteral{3F}{\isacharquery}}x\ {\isaliteral{5C3C696E3E}{\isasymin}}\ range\ {\isaliteral{3F}{\isacharquery}}f{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ True} rewrites
\isa{g\ a\ {\isaliteral{5C3C696E3E}{\isasymin}}\ range\ g} to \isa{True}, but will fail to match
\isa{g{\isaliteral{28}{\isacharparenleft}}h\ b{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{5C3C696E3E}{\isasymin}}\ range{\isaliteral{28}{\isacharparenleft}}{\isaliteral{5C3C6C616D6264613E}{\isasymlambda}}x{\isaliteral{2E}{\isachardot}}\ g{\isaliteral{28}{\isacharparenleft}}h\ x{\isaliteral{29}{\isacharparenright}}{\isaliteral{29}{\isacharparenright}}}.  However, you can
eliminate the offending subterms --- those that are not patterns ---
by adding new variables and conditions.
In our example, we eliminate \isa{{\isaliteral{3F}{\isacharquery}}f\ {\isaliteral{3F}{\isacharquery}}x} and obtain
 \isa{{\isaliteral{3F}{\isacharquery}}y\ {\isaliteral{3D}{\isacharequal}}\ {\isaliteral{3F}{\isacharquery}}f\ {\isaliteral{3F}{\isacharquery}}x\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ {\isaliteral{28}{\isacharparenleft}}{\isaliteral{3F}{\isacharquery}}y\ {\isaliteral{5C3C696E3E}{\isasymin}}\ range\ {\isaliteral{3F}{\isacharquery}}f{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{3D}{\isacharequal}}\ True}, which is fine
as a conditional rewrite rule since conditions can be arbitrary
terms.  However, this trick is not a panacea because the newly
introduced conditions may be hard to solve.
  
There is no restriction on the form of the right-hand
sides.  They may not contain extraneous term or type variables, though.%
\end{isamarkuptext}%
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\isamarkupsubsubsection{The Preprocessor%
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\begin{isamarkuptext}%
\label{sec:simp-preprocessor}
When a theorem is declared a simplification rule, it need not be a
conditional equation already.  The simplifier will turn it into a set of
conditional equations automatically.  For example, \isa{f\ x\ {\isaliteral{3D}{\isacharequal}}\ g\ x\ {\isaliteral{5C3C616E643E}{\isasymand}}\ h\ x\ {\isaliteral{3D}{\isacharequal}}\ k\ x} becomes the two separate
simplification rules \isa{f\ x\ {\isaliteral{3D}{\isacharequal}}\ g\ x} and \isa{h\ x\ {\isaliteral{3D}{\isacharequal}}\ k\ x}. In
general, the input theorem is converted as follows:
\begin{eqnarray}
\neg P &\mapsto& P = \hbox{\isa{False}} \nonumber\\
P \longrightarrow Q &\mapsto& P \Longrightarrow Q \nonumber\\
P \land Q &\mapsto& P,\ Q \nonumber\\
\forall x.~P~x &\mapsto& P~\Var{x}\nonumber\\
\forall x \in A.\ P~x &\mapsto& \Var{x} \in A \Longrightarrow P~\Var{x} \nonumber\\
\isa{if}\ P\ \isa{then}\ Q\ \isa{else}\ R &\mapsto&
 P \Longrightarrow Q,\ \neg P \Longrightarrow R \nonumber
\end{eqnarray}
Once this conversion process is finished, all remaining non-equations
$P$ are turned into trivial equations $P =\isa{True}$.
For example, the formula 
\begin{center}\isa{{\isaliteral{28}{\isacharparenleft}}p\ {\isaliteral{5C3C6C6F6E6772696768746172726F773E}{\isasymlongrightarrow}}\ t\ {\isaliteral{3D}{\isacharequal}}\ u\ {\isaliteral{5C3C616E643E}{\isasymand}}\ {\isaliteral{5C3C6E6F743E}{\isasymnot}}\ r{\isaliteral{29}{\isacharparenright}}\ {\isaliteral{5C3C616E643E}{\isasymand}}\ s}\end{center}
is converted into the three rules
\begin{center}
\isa{p\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ t\ {\isaliteral{3D}{\isacharequal}}\ u},\quad  \isa{p\ {\isaliteral{5C3C4C6F6E6772696768746172726F773E}{\isasymLongrightarrow}}\ r\ {\isaliteral{3D}{\isacharequal}}\ False},\quad  \isa{s\ {\isaliteral{3D}{\isacharequal}}\ True}.
\end{center}
\index{simplification rule|)}
\index{simplification|)}%
\end{isamarkuptext}%
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