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\begin{isabellebody}%
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\def\isabellecontext{simp}%
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%
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\isamarkupsection{Simplification%
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}
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%
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\begin{isamarkuptext}%
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\label{sec:simplification-II}\index{simplification|(}
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This section discusses some additional nifty features not covered so far and
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gives a short introduction to the simplification process itself. The latter
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is helpful to understand why a particular rule does or does not apply in some
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situation.%
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\end{isamarkuptext}%
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%
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\isamarkupsubsection{Advanced features%
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}
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%
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\isamarkupsubsubsection{Congruence rules%
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}
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%
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\begin{isamarkuptext}%
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\label{sec:simp-cong}
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It is hardwired into the simplifier that while simplifying the conclusion $Q$
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of $P \isasymImp Q$ it is legal to make uses of the assumptions $P$. This
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kind of contextual information can also be made available for other
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operators. For example, \isa{xs\ {\isacharequal}\ {\isacharbrackleft}{\isacharbrackright}\ {\isasymlongrightarrow}\ xs\ {\isacharat}\ xs\ {\isacharequal}\ xs} simplifies to \isa{True} because we may use \isa{xs\ {\isacharequal}\ {\isacharbrackleft}{\isacharbrackright}} when simplifying \isa{xs\ {\isacharat}\ xs\ {\isacharequal}\ xs}. The generation of contextual information during simplification is
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controlled by so-called \bfindex{congruence rules}. This is the one for
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\isa{{\isasymlongrightarrow}}:
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\begin{isabelle}%
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\ \ \ \ \ {\isasymlbrakk}P\ {\isacharequal}\ P{\isacharprime}{\isacharsemicolon}\ P{\isacharprime}\ {\isasymLongrightarrow}\ Q\ {\isacharequal}\ Q{\isacharprime}{\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isacharparenleft}P\ {\isasymlongrightarrow}\ Q{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}P{\isacharprime}\ {\isasymlongrightarrow}\ Q{\isacharprime}{\isacharparenright}%
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\end{isabelle}
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It should be read as follows:
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In order to simplify \isa{P\ {\isasymlongrightarrow}\ Q} to \isa{P{\isacharprime}\ {\isasymlongrightarrow}\ Q{\isacharprime}},
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simplify \isa{P} to \isa{P{\isacharprime}}
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and assume \isa{P{\isacharprime}} when simplifying \isa{Q} to \isa{Q{\isacharprime}}.
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Here are some more examples. The congruence rules for bounded
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quantifiers supply contextual information about the bound variable:
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\begin{isabelle}%
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\ \ \ \ \ {\isasymlbrakk}A\ {\isacharequal}\ B{\isacharsemicolon}\ {\isasymAnd}x{\isachardot}\ x\ {\isasymin}\ B\ {\isasymLongrightarrow}\ P\ x\ {\isacharequal}\ Q\ x{\isasymrbrakk}\isanewline
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\ \ \ \ \ {\isasymLongrightarrow}\ {\isacharparenleft}{\isasymforall}x{\isasymin}A{\isachardot}\ P\ x{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}{\isasymforall}x{\isasymin}B{\isachardot}\ Q\ x{\isacharparenright}%
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\end{isabelle}
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The congruence rule for conditional expressions supply contextual
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information for simplifying the arms:
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\begin{isabelle}%
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\ \ \ \ \ {\isasymlbrakk}b\ {\isacharequal}\ c{\isacharsemicolon}\ c\ {\isasymLongrightarrow}\ x\ {\isacharequal}\ u{\isacharsemicolon}\ {\isasymnot}\ c\ {\isasymLongrightarrow}\ y\ {\isacharequal}\ v{\isasymrbrakk}\isanewline
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\ \ \ \ \ {\isasymLongrightarrow}\ {\isacharparenleft}if\ b\ then\ x\ else\ y{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ c\ then\ u\ else\ v{\isacharparenright}%
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\end{isabelle}
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A congruence rule can also \emph{prevent} simplification of some arguments.
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Here is an alternative congruence rule for conditional expressions:
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\begin{isabelle}%
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\ \ \ \ \ b\ {\isacharequal}\ c\ {\isasymLongrightarrow}\ {\isacharparenleft}if\ b\ then\ x\ else\ y{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}if\ c\ then\ x\ else\ y{\isacharparenright}%
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\end{isabelle}
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Only the first argument is simplified; the others remain unchanged.
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This makes simplification much faster and is faithful to the evaluation
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strategy in programming languages, which is why this is the default
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congruence rule for \isa{if}. Analogous rules control the evaluaton of
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\isa{case} expressions.
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You can delare your own congruence rules with the attribute \isa{cong},
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either globally, in the usual manner,
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\begin{quote}
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\isacommand{declare} \textit{theorem-name} \isa{{\isacharbrackleft}cong{\isacharbrackright}}
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\end{quote}
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or locally in a \isa{simp} call by adding the modifier
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\begin{quote}
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\isa{cong{\isacharcolon}} \textit{list of theorem names}
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\end{quote}
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The effect is reversed by \isa{cong\ del} instead of \isa{cong}.
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\begin{warn}
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The congruence rule \isa{conj{\isacharunderscore}cong}
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\begin{isabelle}%
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\ \ \ \ \ {\isasymlbrakk}P\ {\isacharequal}\ P{\isacharprime}{\isacharsemicolon}\ P{\isacharprime}\ {\isasymLongrightarrow}\ Q\ {\isacharequal}\ Q{\isacharprime}{\isasymrbrakk}\ {\isasymLongrightarrow}\ {\isacharparenleft}P\ {\isasymand}\ Q{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}P{\isacharprime}\ {\isasymand}\ Q{\isacharprime}{\isacharparenright}%
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\end{isabelle}
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is occasionally useful but not a default rule; you have to use it explicitly.
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\end{warn}%
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\end{isamarkuptext}%
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%
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\isamarkupsubsubsection{Permutative rewrite rules%
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}
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%
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\begin{isamarkuptext}%
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\index{rewrite rule!permutative|bold}
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\index{rewriting!ordered|bold}
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\index{ordered rewriting|bold}
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\index{simplification!ordered|bold}
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An equation is a \bfindex{permutative rewrite rule} if the left-hand
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side and right-hand side are the same up to renaming of variables. The most
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common permutative rule is commutativity: \isa{x\ {\isacharplus}\ y\ {\isacharequal}\ y\ {\isacharplus}\ x}. Other examples
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include \isa{x\ {\isacharminus}\ y\ {\isacharminus}\ z\ {\isacharequal}\ x\ {\isacharminus}\ z\ {\isacharminus}\ y} in arithmetic and \isa{insert\ x\ {\isacharparenleft}insert\ y\ A{\isacharparenright}\ {\isacharequal}\ insert\ y\ {\isacharparenleft}insert\ x\ A{\isacharparenright}} for sets. Such rules are problematic because
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once they apply, they can be used forever. The simplifier is aware of this
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danger and treats permutative rules by means of a special strategy, called
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\bfindex{ordered rewriting}: a permutative rewrite
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rule is only applied if the term becomes ``smaller'' (with respect to a fixed
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lexicographic ordering on terms). For example, commutativity rewrites
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\isa{b\ {\isacharplus}\ a} to \isa{a\ {\isacharplus}\ b}, but then stops because \isa{a\ {\isacharplus}\ b} is strictly
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smaller than \isa{b\ {\isacharplus}\ a}. Permutative rewrite rules can be turned into
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simplification rules in the usual manner via the \isa{simp} attribute; the
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simplifier recognizes their special status automatically.
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Permutative rewrite rules are most effective in the case of
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associative-commutative functions. (Associativity by itself is not
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permutative.) When dealing with an AC-function~$f$, keep the
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following points in mind:
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\begin{itemize}\index{associative-commutative function}
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\item The associative law must always be oriented from left to right,
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namely $f(f(x,y),z) = f(x,f(y,z))$. The opposite orientation, if
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used with commutativity, can lead to nontermination.
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\item To complete your set of rewrite rules, you must add not just
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associativity~(A) and commutativity~(C) but also a derived rule, {\bf
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left-com\-mut\-ativ\-ity} (LC): $f(x,f(y,z)) = f(y,f(x,z))$.
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\end{itemize}
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Ordered rewriting with the combination of A, C, and LC sorts a term
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lexicographically:
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\[\def\maps#1{~\stackrel{#1}{\leadsto}~}
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f(f(b,c),a) \maps{A} f(b,f(c,a)) \maps{C} f(b,f(a,c)) \maps{LC} f(a,f(b,c)) \]
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Note that ordered rewriting for \isa{{\isacharplus}} and \isa{{\isacharasterisk}} on numbers is rarely
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necessary because the builtin arithmetic capabilities often take care of
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this.%
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\end{isamarkuptext}%
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%
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\isamarkupsubsection{How it works%
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}
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%
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\begin{isamarkuptext}%
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\label{sec:SimpHow}
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Roughly speaking, the simplifier proceeds bottom-up (subterms are simplified
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first) and a conditional equation is only applied if its condition could be
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proved (again by simplification). Below we explain some special features of the rewriting process.%
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\end{isamarkuptext}%
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%
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\isamarkupsubsubsection{Higher-order patterns%
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}
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%
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\begin{isamarkuptext}%
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\index{simplification rule|(}
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So far we have pretended the simplifier can deal with arbitrary
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rewrite rules. This is not quite true. Due to efficiency (and
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potentially also computability) reasons, the simplifier expects the
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left-hand side of each rule to be a so-called \emph{higher-order
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pattern}~\cite{nipkow-patterns}\indexbold{higher-order
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pattern}\indexbold{pattern, higher-order}. This restricts where
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unknowns may occur. Higher-order patterns are terms in $\beta$-normal
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form (this will always be the case unless you have done something
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strange) where each occurrence of an unknown is of the form
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$\Var{f}~x@1~\dots~x@n$, where the $x@i$ are distinct bound
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variables. Thus all ``standard'' rewrite rules, where all unknowns are
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of base type, for example \isa{{\isacharquery}m\ {\isacharplus}\ {\isacharquery}n\ {\isacharplus}\ {\isacharquery}k\ {\isacharequal}\ {\isacharquery}m\ {\isacharplus}\ {\isacharparenleft}{\isacharquery}n\ {\isacharplus}\ {\isacharquery}k{\isacharparenright}}, are OK: if an unknown is
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of base type, it cannot have any arguments. Additionally, the rule
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\isa{{\isacharparenleft}{\isasymforall}x{\isachardot}\ {\isacharquery}P\ x\ {\isasymand}\ {\isacharquery}Q\ x{\isacharparenright}\ {\isacharequal}\ {\isacharparenleft}{\isacharparenleft}{\isasymforall}x{\isachardot}\ {\isacharquery}P\ x{\isacharparenright}\ {\isasymand}\ {\isacharparenleft}{\isasymforall}x{\isachardot}\ {\isacharquery}Q\ x{\isacharparenright}{\isacharparenright}} is also OK, in
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both directions: all arguments of the unknowns \isa{{\isacharquery}P} and
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\isa{{\isacharquery}Q} are distinct bound variables.
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If the left-hand side is not a higher-order pattern, not all is lost
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and the simplifier will still try to apply the rule, but only if it
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matches ``directly'', i.e.\ without much $\lambda$-calculus hocus
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pocus. For example, \isa{{\isacharquery}f\ {\isacharquery}x\ {\isasymin}\ range\ {\isacharquery}f\ {\isacharequal}\ True} rewrites
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\isa{g\ a\ {\isasymin}\ range\ g} to \isa{True}, but will fail to match
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\isa{g{\isacharparenleft}h\ b{\isacharparenright}\ {\isasymin}\ range{\isacharparenleft}{\isasymlambda}x{\isachardot}\ g{\isacharparenleft}h\ x{\isacharparenright}{\isacharparenright}}. However, you can
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replace the offending subterms (in our case \isa{{\isacharquery}f\ {\isacharquery}x}, which
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is not a pattern) by adding new variables and conditions: \isa{{\isacharquery}y\ {\isacharequal}\ {\isacharquery}f\ {\isacharquery}x\ {\isasymLongrightarrow}\ {\isacharquery}y\ {\isasymin}\ range\ {\isacharquery}f\ {\isacharequal}\ True} is fine
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as a conditional rewrite rule since conditions can be arbitrary
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terms. However, this trick is not a panacea because the newly
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introduced conditions may be hard to prove, which has to take place
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before the rule can actually be applied.
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There is basically no restriction on the form of the right-hand
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sides. They may not contain extraneous term or type variables, though.%
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\end{isamarkuptext}%
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%
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\isamarkupsubsubsection{The preprocessor%
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}
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%
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\begin{isamarkuptext}%
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\label{sec:simp-preprocessor}
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When some theorem is declared a simplification rule, it need not be a
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conditional equation already. The simplifier will turn it into a set of
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conditional equations automatically. For example, given \isa{f\ x\ {\isacharequal}\ g\ x\ {\isasymand}\ h\ x\ {\isacharequal}\ k\ x} the simplifier will turn this into the two separate
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simplifiction rules \isa{f\ x\ {\isacharequal}\ g\ x} and \isa{h\ x\ {\isacharequal}\ k\ x}. In
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general, the input theorem is converted as follows:
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\begin{eqnarray}
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\neg P &\mapsto& P = False \nonumber\\
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P \longrightarrow Q &\mapsto& P \Longrightarrow Q \nonumber\\
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P \land Q &\mapsto& P,\ Q \nonumber\\
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\forall x.~P~x &\mapsto& P~\Var{x}\nonumber\\
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\forall x \in A.\ P~x &\mapsto& \Var{x} \in A \Longrightarrow P~\Var{x} \nonumber\\
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\isa{if}\ P\ \isa{then}\ Q\ \isa{else}\ R &\mapsto&
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P \Longrightarrow Q,\ \neg P \Longrightarrow R \nonumber
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\end{eqnarray}
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Once this conversion process is finished, all remaining non-equations
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$P$ are turned into trivial equations $P = True$.
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For example, the formula \begin{center}\isa{{\isacharparenleft}p\ {\isasymlongrightarrow}\ q\ {\isasymand}\ r{\isacharparenright}\ {\isasymand}\ s}\end{center}
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is converted into the three rules
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\begin{center}
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\isa{p\ {\isasymLongrightarrow}\ q\ {\isacharequal}\ True},\quad \isa{p\ {\isasymLongrightarrow}\ r\ {\isacharequal}\ True},\quad \isa{s\ {\isacharequal}\ True}.
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\end{center}
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\index{simplification rule|)}
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\index{simplification|)}%
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\end{isamarkuptext}%
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\end{isabellebody}%
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master: "root"
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%%% End:
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