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%% $Id$


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\chapter{Substitution Tactics} \label{substitution}


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\index{substitution(}


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Replacing equals by equals is a basic form of reasoning. Isabelle supports


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several kinds of equality reasoning. {\bf Substitution} means to replace


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free occurrences of~$t$ by~$u$ in a subgoal. This is easily done, given an


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equality $t=u$, provided the logic possesses the appropriate rule 


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unless you want to substitute even in the assumptions. The tactic


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\ttindex{hyp_subst_tac} performs substitution in the assumptions, but it


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works via objectlevel implication, and therefore must be specially set up


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for each suitable objectlogic.


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Substitution should not be confused with objectlevel {\bf rewriting}.


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Given equalities of the form $t=u$, rewriting replaces instances of~$t$ by


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corresponding instances of~$u$, and continues until it reaches a normal


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form. Substitution handles `oneoff' replacements by particular


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equalities, while rewriting handles general equalities.


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Chapter~\ref{simpchap} discusses Isabelle's rewriting tactics.


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\section{Simple substitution}


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\index{substitution!simple}


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Many logics include a substitution rule of the form\indexbold{*subst}


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$$ \List{\Var{a}=\Var{b}; \Var{P}(\Var{a})} \Imp


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\Var{P}(\Var{b}) \eqno(subst)$$


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In backward proof, this may seem difficult to use: the conclusion


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$\Var{P}(\Var{b})$ admits far too many unifiers. But, if the theorem {\tt


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eqth} asserts $t=u$, then \hbox{\tt eqth RS subst} is the derived rule


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\[ \Var{P}(t) \Imp \Var{P}(u). \]


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Provided $u$ is not an unknown, resolution with this rule is


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wellbehaved.\footnote{Unifying $\Var{P}(u)$ with a formula~$Q$


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expresses~$Q$ in terms of its dependence upon~$u$. There are still $2^k$


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unifiers, if $Q$ has $k$ occurrences of~$u$, but Isabelle ensures that


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the first unifier includes all the occurrences.} To replace $u$ by~$t$ in


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subgoal~$i$, use


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\begin{ttbox}


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resolve_tac [eqth RS subst] \(i\) {\it.}


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\end{ttbox}


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To replace $t$ by~$u$ in


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subgoal~$i$, use


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\begin{ttbox}


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resolve_tac [eqth RS ssubst] \(i\) {\it,}


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\end{ttbox}


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where \ttindexbold{ssubst} is the `swapped' substitution rule


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$$ \List{\Var{a}=\Var{b}; \Var{P}(\Var{b})} \Imp


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\Var{P}(\Var{a}). \eqno(ssubst)$$


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If \ttindex{sym} denotes the symmetry rule


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\(\Var{a}=\Var{b}\Imp\Var{b}=\Var{a}\), then {\tt ssubst} is just


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\hbox{\tt sym RS subst}. Many logics with equality include the rules {\tt


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subst} and {\tt ssubst}, as well as {\tt refl}, {\tt sym} and {\tt trans}


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(for the usual equality laws).


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Elimresolution is wellbehaved with assumptions of the form $t=u$.


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To replace $u$ by~$t$ or $t$ by~$u$ in subgoal~$i$, use


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\begin{ttbox}


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eresolve_tac [subst] \(i\) {\it or} eresolve_tac [ssubst] \(i\) {\it.}


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\end{ttbox}


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\section{Substitution in the hypotheses}


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\index{substitution!in hypotheses}


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Substitution rules, like other rules of natural deduction, do not affect


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the assumptions. This can be inconvenient. Consider proving the subgoal


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\[ \List{c=a; c=b} \Imp a=b. \]


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Calling \hbox{\tt eresolve_tac [ssubst] \(i\)} simply discards the


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assumption~$c=a$, since $c$ does not occur in~$a=b$. Of course, we can


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work out a solution. First apply \hbox{\tt eresolve_tac [subst] \(i\)},


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replacing~$a$ by~$c$:


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\[ \List{c=b} \Imp c=b \]


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Equality reasoning can be difficult, but this trivial proof requires


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nothing more sophisticated than substitution in the assumptions.


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Objectlogics that include the rule~$(subst)$ provide a tactic for this


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purpose:


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\begin{ttbox}


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hyp_subst_tac : int > tactic


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\end{ttbox}


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\begin{description}


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\item[\ttindexbold{hyp_subst_tac} {\it i}]


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selects an equality assumption of the form $t=u$ or $u=t$, where $t$ is a


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free variable or parameter. Deleting this assumption, it replaces $t$


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by~$u$ throughout subgoal~$i$, including the other assumptions.


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\end{description}


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The term being replaced must be a free variable or parameter. Substitution


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for constants is usually unhelpful, since they may appear in other


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theorems. For instance, the best way to use the assumption $0=1$ is to


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contradict a theorem that states $0\not=1$, rather than to replace 0 by~1


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in the subgoal!


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Replacing a free variable causes similar problems if they appear in the


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premises of a rule being derived  the substitution affects objectlevel


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assumptions, not metalevel assumptions. For instance, replacing~$a$


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by~$b$ could make the premise~$P(a)$ worthless. To avoid this problem, call


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\ttindex{cut_facts_tac} to insert the atomic premises as objectlevel


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assumptions.


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\section{Setting up {\tt hyp_subst_tac}}


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Many Isabelle objectlogics, such as {\tt FOL}, {\tt HOL} and their


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descendants, come with {\tt hyp_subst_tac} already defined. A few others,


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such as {\tt CTT}, do not support this tactic because they lack the


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rule~$(subst)$. When defining a new logic that includes a substitution


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rule and implication, you must set up {\tt hyp_subst_tac} yourself. It


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is packaged as the \ML{} functor \ttindex{HypsubstFun}, which takes the


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argument signature~\ttindexbold{HYPSUBST_DATA}:


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\begin{ttbox}


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signature HYPSUBST_DATA =


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sig


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val subst : thm


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val sym : thm


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val rev_cut_eq : thm


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val imp_intr : thm


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val rev_mp : thm


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val dest_eq : term > term*term


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end;


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\end{ttbox}


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Thus, the functor requires the following items:


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\begin{description}


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\item[\ttindexbold{subst}] should be the substitution rule


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$\List{\Var{a}=\Var{b};\; \Var{P}(\Var{a})} \Imp \Var{P}(\Var{b})$.


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\item[\ttindexbold{sym}] should be the symmetry rule


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$\Var{a}=\Var{b}\Imp\Var{b}=\Var{a}$.


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\item[\ttindexbold{rev_cut_eq}] should have the form


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$\List{\Var{a}=\Var{b};\; \Var{a}=\Var{b}\Imp\Var{R}} \Imp \Var{R}$.


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\item[\ttindexbold{imp_intr}] should be the implies introduction


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rule $(\Var{P}\Imp\Var{Q})\Imp \Var{P}\imp\Var{Q}$.


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\item[\ttindexbold{rev_mp}] should be the ``reversed'' implies elimination


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rule $\List{\Var{P}; \;\Var{P}\imp\Var{Q}} \Imp \Var{Q}$.


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\item[\ttindexbold{dest_eq}] should return the pair~$(t,u)$ when


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applied to the \ML{} term that represents~$t=u$. For other terms, it


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should raise exception~\ttindex{Match}.


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\end{description}


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The functor resides in {\tt Provers/hypsubst.ML} on the Isabelle


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distribution directory. It is not sensitive to the precise formalization


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of the objectlogic. It is not concerned with the names of the equality


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and implication symbols, or the types of formula and terms. Coding the


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function {\tt dest_eq} requires knowledge of Isabelle's representation of


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terms. For {\tt FOL} it is defined by


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\begin{ttbox}


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fun dest_eq (Const("Trueprop",_) $ (Const("op =",_)$t$u)) = (t,u);


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\end{ttbox}

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Here {\tt Trueprop} is the coercion from type~$o$ to type~$prop$, while

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\hbox{\tt op =} is the internal name of the infix operator~{\tt=}.


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Patternmatching expresses the function concisely, using wildcards~({\tt_})


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to hide the types.


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Given a subgoal of the form


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\[ \List{P@1; \cdots ; t=u; \cdots ; P@n} \Imp Q, \]


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\ttindexbold{hyp_subst_tac} locates a suitable equality


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assumption and moves it to the last position using elimresolution on {\tt


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rev_cut_eq} (possibly reorienting it using~{\tt sym}):


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\[ \List{P@1; \cdots ; P@n; t=u} \Imp Q \]


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Using $n$ calls of \hbox{\tt eresolve_tac [rev_mp]}, it creates the subgoal


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\[ \List{t=u} \Imp P@1\imp \cdots \imp P@n \imp Q \]


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By \hbox{\tt eresolve_tac [ssubst]}, it replaces~$t$ by~$u$ throughout:


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\[ P'@1\imp \cdots \imp P'@n \imp Q' \]


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Finally, using $n$ calls of \hbox{\tt resolve_tac [imp_intr]}, it restores


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$P'@1$, \ldots, $P'@n$ as assumptions:


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\[ \List{P'@n; \cdots ; P'@1} \Imp Q' \]


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\index{substitution)}
