author | paulson |
Wed, 30 Apr 1997 16:36:59 +0200 | |
changeset 3089 | 32dad29d4666 |
parent 2632 | 1612b99395d4 |
child 3108 | 335efc3f5632 |
permissions | -rw-r--r-- |
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%% $Id$ |
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\chapter{The Classical Reasoner}\label{chap:classical} |
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\index{classical reasoner|(} |
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\newcommand\ainfer[2]{\begin{array}{r@{\,}l}#2\\ \hline#1\end{array}} |
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Although Isabelle is generic, many users will be working in some extension |
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of classical first-order logic. Isabelle's set theory~{\tt ZF} is built |
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upon theory~{\tt FOL}, while higher-order logic contains first-order logic |
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as a fragment. Theorem-proving in predicate logic is undecidable, but many |
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researchers have developed strategies to assist in this task. |
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Isabelle's classical reasoner is an \ML{} functor that accepts certain |
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information about a logic and delivers a suite of automatic tactics. Each |
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tactic takes a collection of rules and executes a simple, non-clausal proof |
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procedure. They are slow and simplistic compared with resolution theorem |
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provers, but they can save considerable time and effort. They can prove |
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theorems such as Pelletier's~\cite{pelletier86} problems~40 and~41 in |
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seconds: |
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\[ (\exists y. \forall x. J(y,x) \bimp \neg J(x,x)) |
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\imp \neg (\forall x. \exists y. \forall z. J(z,y) \bimp \neg J(z,x)) \] |
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\[ (\forall z. \exists y. \forall x. F(x,y) \bimp F(x,z) \conj \neg F(x,x)) |
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\imp \neg (\exists z. \forall x. F(x,z)) |
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\] |
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% |
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The tactics are generic. They are not restricted to first-order logic, and |
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have been heavily used in the development of Isabelle's set theory. Few |
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interactive proof assistants provide this much automation. The tactics can |
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be traced, and their components can be called directly; in this manner, |
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any proof can be viewed interactively. |
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The simplest way to apply the classical reasoner (to subgoal~$i$) is as |
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follows: |
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\begin{ttbox} |
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by (Blast_tac \(i\)); |
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\end{ttbox} |
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If the subgoal is a simple formula of the predicate calculus or set theory, |
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then it should be proved quickly. However, to use the classical reasoner |
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effectively, you need to know how it works and how to choose among the many |
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tactics available, including {\tt Fast_tac} and {\tt Best_tac}. |
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We shall first discuss the underlying principles, then present the classical |
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reasoner. Finally, we shall see how to instantiate it for new logics. The |
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logics {\tt FOL}, {\tt HOL} and {\tt ZF} have it already installed. |
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\section{The sequent calculus} |
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\index{sequent calculus} |
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Isabelle supports natural deduction, which is easy to use for interactive |
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proof. But natural deduction does not easily lend itself to automation, |
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and has a bias towards intuitionism. For certain proofs in classical |
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logic, it can not be called natural. The {\bf sequent calculus}, a |
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generalization of natural deduction, is easier to automate. |
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A {\bf sequent} has the form $\Gamma\turn\Delta$, where $\Gamma$ |
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and~$\Delta$ are sets of formulae.% |
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\footnote{For first-order logic, sequents can equivalently be made from |
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lists or multisets of formulae.} The sequent |
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\[ P@1,\ldots,P@m\turn Q@1,\ldots,Q@n \] |
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is {\bf valid} if $P@1\conj\ldots\conj P@m$ implies $Q@1\disj\ldots\disj |
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Q@n$. Thus $P@1,\ldots,P@m$ represent assumptions, each of which is true, |
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while $Q@1,\ldots,Q@n$ represent alternative goals. A sequent is {\bf |
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basic} if its left and right sides have a common formula, as in $P,Q\turn |
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Q,R$; basic sequents are trivially valid. |
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Sequent rules are classified as {\bf right} or {\bf left}, indicating which |
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side of the $\turn$~symbol they operate on. Rules that operate on the |
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right side are analogous to natural deduction's introduction rules, and |
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left rules are analogous to elimination rules. |
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Recall the natural deduction rules for |
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first-order logic, |
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\iflabelundefined{fol-fig}{from {\it Introduction to Isabelle}}% |
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{Fig.\ts\ref{fol-fig}}. |
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The sequent calculus analogue of~$({\imp}I)$ is the rule |
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$$ \ainfer{\Gamma &\turn \Delta, P\imp Q}{P,\Gamma &\turn \Delta,Q} |
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\eqno({\imp}R) $$ |
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This breaks down some implication on the right side of a sequent; $\Gamma$ |
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and $\Delta$ stand for the sets of formulae that are unaffected by the |
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inference. The analogue of the pair~$({\disj}I1)$ and~$({\disj}I2)$ is the |
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single rule |
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$$ \ainfer{\Gamma &\turn \Delta, P\disj Q}{\Gamma &\turn \Delta,P,Q} |
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\eqno({\disj}R) $$ |
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This breaks down some disjunction on the right side, replacing it by both |
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disjuncts. Thus, the sequent calculus is a kind of multiple-conclusion logic. |
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To illustrate the use of multiple formulae on the right, let us prove |
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the classical theorem $(P\imp Q)\disj(Q\imp P)$. Working backwards, we |
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reduce this formula to a basic sequent: |
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\[ \infer[(\disj)R]{\turn(P\imp Q)\disj(Q\imp P)} |
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{\infer[(\imp)R]{\turn(P\imp Q), (Q\imp P)\;} |
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{\infer[(\imp)R]{P \turn Q, (Q\imp P)\qquad} |
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{P, Q \turn Q, P\qquad\qquad}}} |
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\] |
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This example is typical of the sequent calculus: start with the desired |
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theorem and apply rules backwards in a fairly arbitrary manner. This yields a |
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surprisingly effective proof procedure. Quantifiers add few complications, |
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since Isabelle handles parameters and schematic variables. See Chapter~10 |
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of {\em ML for the Working Programmer}~\cite{paulson91} for further |
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discussion. |
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\section{Simulating sequents by natural deduction} |
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Isabelle can represent sequents directly, as in the object-logic~{\tt LK}\@. |
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But natural deduction is easier to work with, and most object-logics employ |
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it. Fortunately, we can simulate the sequent $P@1,\ldots,P@m\turn |
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Q@1,\ldots,Q@n$ by the Isabelle formula |
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\[ \List{P@1;\ldots;P@m; \neg Q@2;\ldots; \neg Q@n}\Imp Q@1, \] |
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where the order of the assumptions and the choice of~$Q@1$ are arbitrary. |
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Elim-resolution plays a key role in simulating sequent proofs. |
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We can easily handle reasoning on the left. |
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As discussed in |
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\iflabelundefined{destruct}{{\it Introduction to Isabelle}}{\S\ref{destruct}}, |
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elim-resolution with the rules $(\disj E)$, $(\bot E)$ and $(\exists E)$ |
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achieves a similar effect as the corresponding sequent rules. For the |
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other connectives, we use sequent-style elimination rules instead of |
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destruction rules such as $({\conj}E1,2)$ and $(\forall E)$. But note that |
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the rule $(\neg L)$ has no effect under our representation of sequents! |
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$$ \ainfer{\neg P,\Gamma &\turn \Delta}{\Gamma &\turn \Delta,P} |
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\eqno({\neg}L) $$ |
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What about reasoning on the right? Introduction rules can only affect the |
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formula in the conclusion, namely~$Q@1$. The other right-side formulae are |
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represented as negated assumptions, $\neg Q@2$, \ldots,~$\neg Q@n$. |
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\index{assumptions!negated} |
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In order to operate on one of these, it must first be exchanged with~$Q@1$. |
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Elim-resolution with the {\bf swap} rule has this effect: |
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$$ \List{\neg P; \; \neg R\Imp P} \Imp R \eqno(swap)$$ |
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To ensure that swaps occur only when necessary, each introduction rule is |
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converted into a swapped form: it is resolved with the second premise |
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of~$(swap)$. The swapped form of~$({\conj}I)$, which might be |
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called~$({\neg\conj}E)$, is |
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\[ \List{\neg(P\conj Q); \; \neg R\Imp P; \; \neg R\Imp Q} \Imp R. \] |
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Similarly, the swapped form of~$({\imp}I)$ is |
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\[ \List{\neg(P\imp Q); \; \List{\neg R;P}\Imp Q} \Imp R \] |
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Swapped introduction rules are applied using elim-resolution, which deletes |
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the negated formula. Our representation of sequents also requires the use |
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of ordinary introduction rules. If we had no regard for readability, we |
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could treat the right side more uniformly by representing sequents as |
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\[ \List{P@1;\ldots;P@m; \neg Q@1;\ldots; \neg Q@n}\Imp \bot. \] |
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\section{Extra rules for the sequent calculus} |
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As mentioned, destruction rules such as $({\conj}E1,2)$ and $(\forall E)$ |
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must be replaced by sequent-style elimination rules. In addition, we need |
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rules to embody the classical equivalence between $P\imp Q$ and $\neg P\disj |
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Q$. The introduction rules~$({\disj}I1,2)$ are replaced by a rule that |
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simulates $({\disj}R)$: |
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\[ (\neg Q\Imp P) \Imp P\disj Q \] |
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The destruction rule $({\imp}E)$ is replaced by |
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\[ \List{P\imp Q;\; \neg P\Imp R;\; Q\Imp R} \Imp R. \] |
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Quantifier replication also requires special rules. In classical logic, |
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$\exists x{.}P$ is equivalent to $\neg\forall x{.}\neg P$; the rules |
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$(\exists R)$ and $(\forall L)$ are dual: |
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\[ \ainfer{\Gamma &\turn \Delta, \exists x{.}P} |
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{\Gamma &\turn \Delta, \exists x{.}P, P[t/x]} \; (\exists R) |
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\qquad |
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\ainfer{\forall x{.}P, \Gamma &\turn \Delta} |
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{P[t/x], \forall x{.}P, \Gamma &\turn \Delta} \; (\forall L) |
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\] |
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Thus both kinds of quantifier may be replicated. Theorems requiring |
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multiple uses of a universal formula are easy to invent; consider |
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\[ (\forall x.P(x)\imp P(f(x))) \conj P(a) \imp P(f^n(a)), \] |
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for any~$n>1$. Natural examples of the multiple use of an existential |
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formula are rare; a standard one is $\exists x.\forall y. P(x)\imp P(y)$. |
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Forgoing quantifier replication loses completeness, but gains decidability, |
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since the search space becomes finite. Many useful theorems can be proved |
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without replication, and the search generally delivers its verdict in a |
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reasonable time. To adopt this approach, represent the sequent rules |
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$(\exists R)$, $(\exists L)$ and $(\forall R)$ by $(\exists I)$, $(\exists |
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E)$ and $(\forall I)$, respectively, and put $(\forall E)$ into elimination |
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form: |
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$$ \List{\forall x{.}P(x); P(t)\Imp Q} \Imp Q \eqno(\forall E@2) $$ |
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Elim-resolution with this rule will delete the universal formula after a |
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single use. To replicate universal quantifiers, replace the rule by |
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$$ \List{\forall x{.}P(x);\; \List{P(t); \forall x{.}P(x)}\Imp Q} \Imp Q. |
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\eqno(\forall E@3) $$ |
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To replicate existential quantifiers, replace $(\exists I)$ by |
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\[ \List{\neg(\exists x{.}P(x)) \Imp P(t)} \Imp \exists x{.}P(x). \] |
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All introduction rules mentioned above are also useful in swapped form. |
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Replication makes the search space infinite; we must apply the rules with |
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care. The classical reasoner distinguishes between safe and unsafe |
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rules, applying the latter only when there is no alternative. Depth-first |
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search may well go down a blind alley; best-first search is better behaved |
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in an infinite search space. However, quantifier replication is too |
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expensive to prove any but the simplest theorems. |
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\section{Classical rule sets} |
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\index{classical sets} |
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Each automatic tactic takes a {\bf classical set} --- a collection of |
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rules, classified as introduction or elimination and as {\bf safe} or {\bf |
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unsafe}. In general, safe rules can be attempted blindly, while unsafe |
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rules must be used with care. A safe rule must never reduce a provable |
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goal to an unprovable set of subgoals. |
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The rule~$({\disj}I1)$ is unsafe because it reduces $P\disj Q$ to~$P$. Any |
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rule is unsafe whose premises contain new unknowns. The elimination |
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rule~$(\forall E@2)$ is unsafe, since it is applied via elim-resolution, |
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which discards the assumption $\forall x{.}P(x)$ and replaces it by the |
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weaker assumption~$P(\Var{t})$. The rule $({\exists}I)$ is unsafe for |
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similar reasons. The rule~$(\forall E@3)$ is unsafe in a different sense: |
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since it keeps the assumption $\forall x{.}P(x)$, it is prone to looping. |
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In classical first-order logic, all rules are safe except those mentioned |
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above. |
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The safe/unsafe distinction is vague, and may be regarded merely as a way |
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of giving some rules priority over others. One could argue that |
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$({\disj}E)$ is unsafe, because repeated application of it could generate |
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exponentially many subgoals. Induction rules are unsafe because inductive |
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proofs are difficult to set up automatically. Any inference is unsafe that |
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instantiates an unknown in the proof state --- thus \ttindex{match_tac} |
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must be used, rather than \ttindex{resolve_tac}. Even proof by assumption |
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is unsafe if it instantiates unknowns shared with other subgoals --- thus |
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\ttindex{eq_assume_tac} must be used, rather than \ttindex{assume_tac}. |
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\subsection{Adding rules to classical sets} |
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Classical rule sets belong to the abstract type \mltydx{claset}, which |
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supports the following operations (provided the classical reasoner is |
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installed!): |
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\begin{ttbox} |
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empty_cs : claset |
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print_cs : claset -> unit |
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addSIs : claset * thm list -> claset \hfill{\bf infix 4} |
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addSEs : claset * thm list -> claset \hfill{\bf infix 4} |
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addSDs : claset * thm list -> claset \hfill{\bf infix 4} |
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addIs : claset * thm list -> claset \hfill{\bf infix 4} |
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addEs : claset * thm list -> claset \hfill{\bf infix 4} |
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addDs : claset * thm list -> claset \hfill{\bf infix 4} |
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delrules : claset * thm list -> claset \hfill{\bf infix 4} |
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\end{ttbox} |
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The add operations ignore any rule already present in the claset with the same |
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classification (such as Safe Introduction). They print a warning if the rule |
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has already been added with some other classification, but add the rule |
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anyway. Calling {\tt delrules} deletes all occurrences of a rule from the |
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claset, but see the warning below concerning destruction rules. |
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\begin{ttdescription} |
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\item[\ttindexbold{empty_cs}] is the empty classical set. |
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\item[\ttindexbold{print_cs} $cs$] prints the rules of~$cs$. |
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\item[$cs$ addSIs $rules$] \indexbold{*addSIs} |
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adds safe introduction~$rules$ to~$cs$. |
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\item[$cs$ addSEs $rules$] \indexbold{*addSEs} |
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adds safe elimination~$rules$ to~$cs$. |
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\item[$cs$ addSDs $rules$] \indexbold{*addSDs} |
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adds safe destruction~$rules$ to~$cs$. |
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\item[$cs$ addIs $rules$] \indexbold{*addIs} |
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adds unsafe introduction~$rules$ to~$cs$. |
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\item[$cs$ addEs $rules$] \indexbold{*addEs} |
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adds unsafe elimination~$rules$ to~$cs$. |
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\item[$cs$ addDs $rules$] \indexbold{*addDs} |
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adds unsafe destruction~$rules$ to~$cs$. |
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\item[$cs$ delrules $rules$] \indexbold{*delrules} |
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deletes~$rules$ from~$cs$. It prints a warning for those rules that are not |
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in~$cs$. |
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\end{ttdescription} |
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\begin{warn} |
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If you added $rule$ using {\tt addSDs} or {\tt addDs}, then you must delete |
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it as follows: |
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\begin{ttbox} |
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\(cs\) delrules [make_elim \(rule\)] |
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\end{ttbox} |
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\par\noindent |
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This is necessary because the operators {\tt addSDs} and {\tt addDs} convert |
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the destruction rules to elimination rules by applying \ttindex{make_elim}, |
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and then insert them using {\tt addSEs} and {\tt addEs}, respectively. |
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\end{warn} |
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Introduction rules are those that can be applied using ordinary resolution. |
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The classical set automatically generates their swapped forms, which will |
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be applied using elim-resolution. Elimination rules are applied using |
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elim-resolution. In a classical set, rules are sorted by the number of new |
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subgoals they will yield; rules that generate the fewest subgoals will be |
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tried first (see \S\ref{biresolve_tac}). |
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\subsection{Modifying the search step} |
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For a given classical set, the proof strategy is simple. Perform as many |
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safe inferences as possible; or else, apply certain safe rules, allowing |
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instantiation of unknowns; or else, apply an unsafe rule. The tactics may |
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also apply {\tt hyp_subst_tac}, if they have been set up to do so (see |
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below). They may perform a form of Modus Ponens: if there are assumptions |
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$P\imp Q$ and~$P$, then replace $P\imp Q$ by~$Q$. |
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The classical reasoning tactics---except {\tt blast_tac}!---allow you to |
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modify this basic proof strategy by |
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applying two arbitrary {\bf wrapper tacticals} to it. This affects each step of |
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the search. Usually they are the identity tacticals, but they could apply |
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another tactic before or after the step tactic. The first one, which is |
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considered to be safe, affects \ttindex{safe_step_tac} and all the tactics that |
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call it. The the second one, which may be unsafe, affects |
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\ttindex{step_tac}, \ttindex{slow_step_tac} and the tactics that call them. |
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\begin{ttbox} |
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addss : claset * simpset -> claset \hfill{\bf infix 4} |
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addSbefore : claset * (int -> tactic) -> claset \hfill{\bf infix 4} |
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addSaltern : claset * (int -> tactic) -> claset \hfill{\bf infix 4} |
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setSWrapper : claset * ((int -> tactic) -> |
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(int -> tactic)) -> claset \hfill{\bf infix 4} |
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compSWrapper : claset * ((int -> tactic) -> |
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(int -> tactic)) -> claset \hfill{\bf infix 4} |
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addbefore : claset * (int -> tactic) -> claset \hfill{\bf infix 4} |
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addaltern : claset * (int -> tactic) -> claset \hfill{\bf infix 4} |
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setWrapper : claset * ((int -> tactic) -> |
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(int -> tactic)) -> claset \hfill{\bf infix 4} |
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compWrapper : claset * ((int -> tactic) -> |
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(int -> tactic)) -> claset \hfill{\bf infix 4} |
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\end{ttbox} |
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% |
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\index{simplification!from classical reasoner} |
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The wrapper tacticals underly the operator \ttindex{addss}, which combines |
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each search step by simplification. Strictly speaking, {\tt addss} is not |
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part of the classical reasoner. It should be defined (using {\tt addSaltern (CHANGED o (safe_asm_more_full_simp_tac ss)}) when the simplifier is installed. |
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\begin{ttdescription} |
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\item[$cs$ addss $ss$] \indexbold{*addss} |
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adds the simpset~$ss$ to the classical set. The assumptions and goal will be |
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simplified, in a safe way, after the safe steps of the search. |
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|
327 |
|
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|
328 |
\item[$cs$ addSbefore $tac$] \indexbold{*addSbefore} |
5e5c78ba955e
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diff
changeset
|
329 |
changes the safe wrapper tactical to apply the given tactic {\em before} |
5e5c78ba955e
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oheimb
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2479
diff
changeset
|
330 |
each safe step of the search. |
5e5c78ba955e
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|
331 |
|
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|
332 |
\item[$cs$ addSaltern $tac$] \indexbold{*addSaltern} |
5e5c78ba955e
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oheimb
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2479
diff
changeset
|
333 |
changes the safe wrapper tactical to apply the given tactic when a safe step |
5e5c78ba955e
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|
334 |
of the search would fail. |
5e5c78ba955e
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|
335 |
|
5e5c78ba955e
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|
336 |
\item[$cs$ setSWrapper $tactical$] \indexbold{*setSWrapper} |
5e5c78ba955e
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diff
changeset
|
337 |
specifies a new safe wrapper tactical. |
5e5c78ba955e
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|
338 |
|
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diff
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|
339 |
\item[$cs$ compSWrapper $tactical$] \indexbold{*compSWrapper} |
5e5c78ba955e
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2479
diff
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|
340 |
composes the $tactical$ with the existing safe wrapper tactical, |
5e5c78ba955e
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|
341 |
to combine their effects. |
1099 | 342 |
|
343 |
\item[$cs$ addbefore $tac$] \indexbold{*addbefore} |
|
2631
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|
344 |
changes the (unsafe) wrapper tactical to apply the given tactic, which should |
5e5c78ba955e
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|
345 |
be safe, {\em before} each step of the search. |
1099 | 346 |
|
2631
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|
347 |
\item[$cs$ addaltern $tac$] \indexbold{*addaltern} |
5e5c78ba955e
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diff
changeset
|
348 |
changes the (unsafe) wrapper tactical to apply the given tactic |
5e5c78ba955e
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oheimb
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diff
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|
349 |
{\em alternatively} after each step of the search. |
1099 | 350 |
|
2631
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|
351 |
\item[$cs$ setWrapper $tactical$] \indexbold{*setWrapper} |
5e5c78ba955e
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diff
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|
352 |
specifies a new (unsafe) wrapper tactical. |
1099 | 353 |
|
2631
5e5c78ba955e
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oheimb
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|
354 |
\item[$cs$ compWrapper $tactical$] \indexbold{*compWrapper} |
5e5c78ba955e
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diff
changeset
|
355 |
composes the $tactical$ with the existing (unsafe) wrapper tactical, |
5e5c78ba955e
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changeset
|
356 |
to combine their effects. |
1099 | 357 |
\end{ttdescription} |
358 |
||
104 | 359 |
|
360 |
\section{The classical tactics} |
|
319 | 361 |
\index{classical reasoner!tactics} |
104 | 362 |
If installed, the classical module provides several tactics (and other |
363 |
operations) for simulating the classical sequent calculus. |
|
364 |
||
3089 | 365 |
\subsection{The automatic tableau prover} |
366 |
The tactic {\tt blast_tac} finds a proof rapidly using a separate tableau |
|
367 |
prover and then reconstructs the proof using Isabelle tactics. It is much |
|
368 |
faster and more powerful than the other classical reasoning tactics, but has |
|
369 |
major limitations too. |
|
370 |
\begin{itemize} |
|
371 |
\item It does not use the wrapper tacticals described above, such as |
|
372 |
\ttindex{addss}. |
|
373 |
\item It ignores types, which can cause problems in \HOL. If it applies a rule |
|
374 |
whose types are inappropriate, then proof reconstruction will fail. |
|
375 |
\item It does not perform higher-order unification, as needed by the rule {\tt |
|
376 |
rangeI} in {\HOL} and {\tt RepFunI} in {\ZF}. There are often |
|
377 |
alternatives to such rules, for example {\tt |
|
378 |
range_eqI} and {\tt RepFun_eqI}. |
|
379 |
\item The message {\small\tt Function Var's argument not a bound variable\ } |
|
380 |
relates to the lack of higher-order unification. Function variables |
|
381 |
may only be applied to parameters of the subgoal. |
|
382 |
\item Its proof strategy is more general than {\tt fast_tac}'s but can be |
|
383 |
slower. If {\tt blast_tac} fails or seems to be running forever, try {\tt |
|
384 |
fast_tac} and the other tactics described below. |
|
385 |
\end{itemize} |
|
386 |
% |
|
387 |
\begin{ttbox} |
|
388 |
blast_tac : claset -> int -> tactic |
|
389 |
Blast.depth_tac : claset -> int -> int -> tactic |
|
390 |
Blast.trace : bool ref \hfill{\bf initially false} |
|
391 |
\end{ttbox} |
|
392 |
The two tactics differ on how they bound the number of unsafe steps used in a |
|
393 |
proof. While {\tt blast_tac} starts with a bound of zero and increases it |
|
394 |
successively to~20, {\tt Blast.depth_tac} applies a user-supplied search bound. |
|
395 |
\begin{ttdescription} |
|
396 |
\item[\ttindexbold{blast_tac} $cs$ $i$] tries to prove |
|
397 |
subgoal~$i$ using iterative deepening to increase the search bound. |
|
398 |
||
399 |
\item[\ttindexbold{Blast.depth_tac} $cs$ $lim$ $i$] tries |
|
400 |
to prove subgoal~$i$ using a search bound of $lim$. Often a slow |
|
401 |
proof using {\tt blast_tac} can be made much faster by supplying the |
|
402 |
successful search bound to this tactic instead. |
|
403 |
||
404 |
\item[\ttindexbold{Blast.trace} := true;] \index{tracing!of classical prover} |
|
405 |
causes the tableau prover to print a trace of its search. At each step it |
|
406 |
displays the formula currently being examined and reports whether the branch |
|
407 |
has been closed, extended or split. |
|
408 |
\end{ttdescription} |
|
409 |
||
332 | 410 |
\subsection{The automatic tactics} |
411 |
\begin{ttbox} |
|
875
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332
diff
changeset
|
412 |
fast_tac : claset -> int -> tactic |
a0b71a4bbe5e
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lcp
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332
diff
changeset
|
413 |
best_tac : claset -> int -> tactic |
a0b71a4bbe5e
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332
diff
changeset
|
414 |
slow_tac : claset -> int -> tactic |
a0b71a4bbe5e
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332
diff
changeset
|
415 |
slow_best_tac : claset -> int -> tactic |
332 | 416 |
\end{ttbox} |
875
a0b71a4bbe5e
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332
diff
changeset
|
417 |
These tactics work by applying {\tt step_tac} or {\tt slow_step_tac} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
418 |
repeatedly. Their effect is restricted (by {\tt SELECT_GOAL}) to one subgoal; |
3089 | 419 |
they either prove this subgoal or fail. The {\tt slow_} versions are more |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
420 |
powerful but can be much slower. |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
421 |
|
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
422 |
The best-first tactics are guided by a heuristic function: typically, the |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
423 |
total size of the proof state. This function is supplied in the functor call |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
424 |
that sets up the classical reasoner. |
332 | 425 |
\begin{ttdescription} |
426 |
\item[\ttindexbold{fast_tac} $cs$ $i$] applies {\tt step_tac} using |
|
3089 | 427 |
depth-first search, to prove subgoal~$i$. |
332 | 428 |
|
429 |
\item[\ttindexbold{best_tac} $cs$ $i$] applies {\tt step_tac} using |
|
3089 | 430 |
best-first search, to prove subgoal~$i$. |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
431 |
|
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
432 |
\item[\ttindexbold{slow_tac} $cs$ $i$] applies {\tt slow_step_tac} using |
3089 | 433 |
depth-first search, to prove subgoal~$i$. |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
434 |
|
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
435 |
\item[\ttindexbold{slow_best_tac} $cs$ $i$] applies {\tt slow_step_tac} using |
3089 | 436 |
best-first search, to prove subgoal~$i$. |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
437 |
\end{ttdescription} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
438 |
|
a0b71a4bbe5e
documented slow_tac, slow_best_tac, depth_tac, deepen_tac
lcp
parents:
332
diff
changeset
|
439 |
|
a0b71a4bbe5e
documented slow_tac, slow_best_tac, depth_tac, deepen_tac
lcp
parents:
332
diff
changeset
|
440 |
\subsection{Depth-limited tactics} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
441 |
\begin{ttbox} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
442 |
depth_tac : claset -> int -> int -> tactic |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
443 |
deepen_tac : claset -> int -> int -> tactic |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
444 |
\end{ttbox} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
445 |
These work by exhaustive search up to a specified depth. Unsafe rules are |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
446 |
modified to preserve the formula they act on, so that it be used repeatedly. |
1099 | 447 |
They can prove more goals than {\tt fast_tac} can but are much |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
448 |
slower, for example if the assumptions have many universal quantifiers. |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
449 |
|
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
450 |
The depth limits the number of unsafe steps. If you can estimate the minimum |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
451 |
number of unsafe steps needed, supply this value as~$m$ to save time. |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
452 |
\begin{ttdescription} |
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
453 |
\item[\ttindexbold{depth_tac} $cs$ $m$ $i$] |
3089 | 454 |
tries to prove subgoal~$i$ by exhaustive search up to depth~$m$. |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
455 |
|
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
456 |
\item[\ttindexbold{deepen_tac} $cs$ $m$ $i$] |
3089 | 457 |
tries to prove subgoal~$i$ by iterative deepening. It calls {\tt depth_tac} |
875
a0b71a4bbe5e
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lcp
parents:
332
diff
changeset
|
458 |
repeatedly with increasing depths, starting with~$m$. |
332 | 459 |
\end{ttdescription} |
460 |
||
461 |
||
104 | 462 |
\subsection{Single-step tactics} |
463 |
\begin{ttbox} |
|
464 |
safe_step_tac : claset -> int -> tactic |
|
465 |
safe_tac : claset -> tactic |
|
466 |
inst_step_tac : claset -> int -> tactic |
|
467 |
step_tac : claset -> int -> tactic |
|
468 |
slow_step_tac : claset -> int -> tactic |
|
469 |
\end{ttbox} |
|
470 |
The automatic proof procedures call these tactics. By calling them |
|
471 |
yourself, you can execute these procedures one step at a time. |
|
308 | 472 |
\begin{ttdescription} |
104 | 473 |
\item[\ttindexbold{safe_step_tac} $cs$ $i$] performs a safe step on |
2631
5e5c78ba955e
description of safe vs. unsafe wrapper and the functions involved
oheimb
parents:
2479
diff
changeset
|
474 |
subgoal~$i$. The safe wrapper tactical is applied to a tactic that may include |
5e5c78ba955e
description of safe vs. unsafe wrapper and the functions involved
oheimb
parents:
2479
diff
changeset
|
475 |
proof by assumption or Modus Ponens (taking care not to instantiate unknowns), |
5e5c78ba955e
description of safe vs. unsafe wrapper and the functions involved
oheimb
parents:
2479
diff
changeset
|
476 |
or {\tt hyp_subst_tac}. |
104 | 477 |
|
478 |
\item[\ttindexbold{safe_tac} $cs$] repeatedly performs safe steps on all |
|
479 |
subgoals. It is deterministic, with at most one outcome. If the automatic |
|
480 |
tactics fail, try using {\tt safe_tac} to open up your formula; then you |
|
481 |
can replicate certain quantifiers explicitly by applying appropriate rules. |
|
482 |
||
483 |
\item[\ttindexbold{inst_step_tac} $cs$ $i$] is like {\tt safe_step_tac}, |
|
484 |
but allows unknowns to be instantiated. |
|
485 |
||
1099 | 486 |
\item[\ttindexbold{step_tac} $cs$ $i$] is the basic step of the proof |
2631
5e5c78ba955e
description of safe vs. unsafe wrapper and the functions involved
oheimb
parents:
2479
diff
changeset
|
487 |
procedure. The (unsafe) wrapper tactical is applied to a tactic that tries |
5e5c78ba955e
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oheimb
parents:
2479
diff
changeset
|
488 |
{\tt safe_tac}, {\tt inst_step_tac}, or applies an unsafe rule from~$cs$. |
104 | 489 |
|
490 |
\item[\ttindexbold{slow_step_tac}] |
|
491 |
resembles {\tt step_tac}, but allows backtracking between using safe |
|
492 |
rules with instantiation ({\tt inst_step_tac}) and using unsafe rules. |
|
875
a0b71a4bbe5e
documented slow_tac, slow_best_tac, depth_tac, deepen_tac
lcp
parents:
332
diff
changeset
|
493 |
The resulting search space is larger. |
308 | 494 |
\end{ttdescription} |
104 | 495 |
|
1869 | 496 |
\subsection{The current claset} |
2479 | 497 |
Some logics (\FOL, {\HOL} and \ZF) support the concept of a current |
498 |
claset\index{claset!current}. This is a default set of classical rules. The |
|
499 |
underlying idea is quite similar to that of a current simpset described in |
|
500 |
\S\ref{sec:simp-for-dummies}; please read that section, including its |
|
501 |
warnings. Just like simpsets, clasets can be associated with theories. The |
|
502 |
tactics |
|
1869 | 503 |
\begin{ttbox} |
3089 | 504 |
Blast_tac : int -> tactic |
1869 | 505 |
Fast_tac : int -> tactic |
506 |
Best_tac : int -> tactic |
|
507 |
Deepen_tac : int -> int -> tactic |
|
3089 | 508 |
Step_tac : int -> tactic |
1869 | 509 |
\end{ttbox} |
3089 | 510 |
\indexbold{*Blast_tac}\indexbold{*Best_tac}\indexbold{*Fast_tac}% |
511 |
\indexbold{*Deepen_tac}\indexbold{*Step_tac} |
|
512 |
make use of the current claset. E.g. {\tt Blast_tac} is defined as follows: |
|
1869 | 513 |
\begin{ttbox} |
3089 | 514 |
fun Blast_tac i = fast_tac (!claset) i; |
1869 | 515 |
\end{ttbox} |
516 |
where \ttindex{!claset} is the current claset. |
|
517 |
The functions |
|
518 |
\begin{ttbox} |
|
519 |
AddSIs, AddSEs, AddSDs, AddIs, AddEs, AddDs: thm list -> unit |
|
520 |
\end{ttbox} |
|
521 |
\indexbold{*AddSIs} \indexbold{*AddSEs} \indexbold{*AddSDs} |
|
522 |
\indexbold{*AddIs} \indexbold{*AddEs} \indexbold{*AddDs} |
|
523 |
are used to add rules to the current claset. They work exactly like their |
|
524 |
lower case counterparts {\tt addSIs} etc. |
|
525 |
\begin{ttbox} |
|
526 |
Delrules : thm list -> unit |
|
527 |
\end{ttbox} |
|
528 |
deletes rules from the current claset. You do not need to worry via which of |
|
529 |
the above {\tt Add} functions the rule was initially added. |
|
104 | 530 |
|
531 |
\subsection{Other useful tactics} |
|
319 | 532 |
\index{tactics!for contradiction} |
533 |
\index{tactics!for Modus Ponens} |
|
104 | 534 |
\begin{ttbox} |
535 |
contr_tac : int -> tactic |
|
536 |
mp_tac : int -> tactic |
|
537 |
eq_mp_tac : int -> tactic |
|
538 |
swap_res_tac : thm list -> int -> tactic |
|
539 |
\end{ttbox} |
|
540 |
These can be used in the body of a specialized search. |
|
308 | 541 |
\begin{ttdescription} |
319 | 542 |
\item[\ttindexbold{contr_tac} {\it i}]\index{assumptions!contradictory} |
543 |
solves subgoal~$i$ by detecting a contradiction among two assumptions of |
|
544 |
the form $P$ and~$\neg P$, or fail. It may instantiate unknowns. The |
|
545 |
tactic can produce multiple outcomes, enumerating all possible |
|
546 |
contradictions. |
|
104 | 547 |
|
548 |
\item[\ttindexbold{mp_tac} {\it i}] |
|
549 |
is like {\tt contr_tac}, but also attempts to perform Modus Ponens in |
|
550 |
subgoal~$i$. If there are assumptions $P\imp Q$ and~$P$, then it replaces |
|
551 |
$P\imp Q$ by~$Q$. It may instantiate unknowns. It fails if it can do |
|
552 |
nothing. |
|
553 |
||
554 |
\item[\ttindexbold{eq_mp_tac} {\it i}] |
|
555 |
is like {\tt mp_tac} {\it i}, but may not instantiate unknowns --- thus, it |
|
556 |
is safe. |
|
557 |
||
558 |
\item[\ttindexbold{swap_res_tac} {\it thms} {\it i}] refines subgoal~$i$ of |
|
559 |
the proof state using {\it thms}, which should be a list of introduction |
|
3089 | 560 |
rules. First, it attempts to prove the goal using {\tt assume_tac} or |
104 | 561 |
{\tt contr_tac}. It then attempts to apply each rule in turn, attempting |
562 |
resolution and also elim-resolution with the swapped form. |
|
308 | 563 |
\end{ttdescription} |
104 | 564 |
|
565 |
\subsection{Creating swapped rules} |
|
566 |
\begin{ttbox} |
|
567 |
swapify : thm list -> thm list |
|
568 |
joinrules : thm list * thm list -> (bool * thm) list |
|
569 |
\end{ttbox} |
|
308 | 570 |
\begin{ttdescription} |
104 | 571 |
\item[\ttindexbold{swapify} {\it thms}] returns a list consisting of the |
572 |
swapped versions of~{\it thms}, regarded as introduction rules. |
|
573 |
||
308 | 574 |
\item[\ttindexbold{joinrules} ({\it intrs}, {\it elims})] |
104 | 575 |
joins introduction rules, their swapped versions, and elimination rules for |
576 |
use with \ttindex{biresolve_tac}. Each rule is paired with~{\tt false} |
|
577 |
(indicating ordinary resolution) or~{\tt true} (indicating |
|
578 |
elim-resolution). |
|
308 | 579 |
\end{ttdescription} |
104 | 580 |
|
581 |
||
286 | 582 |
\section{Setting up the classical reasoner} |
319 | 583 |
\index{classical reasoner!setting up} |
104 | 584 |
Isabelle's classical object-logics, including {\tt FOL} and {\tt HOL}, have |
286 | 585 |
the classical reasoner already set up. When defining a new classical logic, |
586 |
you should set up the reasoner yourself. It consists of the \ML{} functor |
|
104 | 587 |
\ttindex{ClassicalFun}, which takes the argument |
319 | 588 |
signature {\tt CLASSICAL_DATA}: |
104 | 589 |
\begin{ttbox} |
590 |
signature CLASSICAL_DATA = |
|
591 |
sig |
|
592 |
val mp : thm |
|
593 |
val not_elim : thm |
|
594 |
val swap : thm |
|
595 |
val sizef : thm -> int |
|
596 |
val hyp_subst_tacs : (int -> tactic) list |
|
597 |
end; |
|
598 |
\end{ttbox} |
|
599 |
Thus, the functor requires the following items: |
|
308 | 600 |
\begin{ttdescription} |
319 | 601 |
\item[\tdxbold{mp}] should be the Modus Ponens rule |
104 | 602 |
$\List{\Var{P}\imp\Var{Q};\; \Var{P}} \Imp \Var{Q}$. |
603 |
||
319 | 604 |
\item[\tdxbold{not_elim}] should be the contradiction rule |
104 | 605 |
$\List{\neg\Var{P};\; \Var{P}} \Imp \Var{R}$. |
606 |
||
319 | 607 |
\item[\tdxbold{swap}] should be the swap rule |
104 | 608 |
$\List{\neg \Var{P}; \; \neg \Var{R}\Imp \Var{P}} \Imp \Var{R}$. |
609 |
||
610 |
\item[\ttindexbold{sizef}] is the heuristic function used for best-first |
|
611 |
search. It should estimate the size of the remaining subgoals. A good |
|
612 |
heuristic function is \ttindex{size_of_thm}, which measures the size of the |
|
613 |
proof state. Another size function might ignore certain subgoals (say, |
|
614 |
those concerned with type checking). A heuristic function might simply |
|
615 |
count the subgoals. |
|
616 |
||
319 | 617 |
\item[\ttindexbold{hyp_subst_tacs}] is a list of tactics for substitution in |
104 | 618 |
the hypotheses, typically created by \ttindex{HypsubstFun} (see |
619 |
Chapter~\ref{substitution}). This list can, of course, be empty. The |
|
620 |
tactics are assumed to be safe! |
|
308 | 621 |
\end{ttdescription} |
104 | 622 |
The functor is not at all sensitive to the formalization of the |
623 |
object-logic. It does not even examine the rules, but merely applies them |
|
624 |
according to its fixed strategy. The functor resides in {\tt |
|
319 | 625 |
Provers/classical.ML} in the Isabelle distribution directory. |
104 | 626 |
|
319 | 627 |
\index{classical reasoner|)} |