doc-src/Ref/classical.tex
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classical atts now intro! / intro / intro?;
1 %% $Id$
2 \chapter{The Classical Reasoner}\label{chap:classical}
3 \index{classical reasoner|(}
4 \newcommand\ainfer{\begin{array}{r@{\,}l}#2\\ \hline#1\end{array}}
6 Although Isabelle is generic, many users will be working in some
7 extension of classical first-order logic.
8 Isabelle's set theory~{\tt ZF} is built upon theory~\texttt{FOL},
9 while {\HOL} conceptually contains first-order logic as a fragment.
10 Theorem-proving in predicate logic is undecidable, but many
11 researchers have developed strategies to assist in this task.
13 Isabelle's classical reasoner is an \ML{} functor that accepts certain
14 information about a logic and delivers a suite of automatic tactics.  Each
15 tactic takes a collection of rules and executes a simple, non-clausal proof
16 procedure.  They are slow and simplistic compared with resolution theorem
17 provers, but they can save considerable time and effort.  They can prove
18 theorems such as Pelletier's~\cite{pelletier86} problems~40 and~41 in
19 seconds:
20 $(\exists y. \forall x. J(y,x) \bimp \neg J(x,x)) 21 \imp \neg (\forall x. \exists y. \forall z. J(z,y) \bimp \neg J(z,x))$
22 $(\forall z. \exists y. \forall x. F(x,y) \bimp F(x,z) \conj \neg F(x,x)) 23 \imp \neg (\exists z. \forall x. F(x,z)) 24$
25 %
26 The tactics are generic.  They are not restricted to first-order logic, and
27 have been heavily used in the development of Isabelle's set theory.  Few
28 interactive proof assistants provide this much automation.  The tactics can
29 be traced, and their components can be called directly; in this manner,
30 any proof can be viewed interactively.
32 The simplest way to apply the classical reasoner (to subgoal~$i$) is to type
33 \begin{ttbox}
34 by (Blast_tac $$i$$);
35 \end{ttbox}
36 This command quickly proves most simple formulas of the predicate calculus or
37 set theory.  To attempt to prove subgoals using a combination of
38 rewriting and classical reasoning, try
39 \begin{ttbox}
40 auto();                         \emph{\textrm{applies to all subgoals}}
41 force i;                        \emph{\textrm{applies to one subgoal}}
42 \end{ttbox}
43 To do all obvious logical steps, even if they do not prove the
44 subgoal, type one of the following:
45 \begin{ttbox}
46 by Safe_tac;                   \emph{\textrm{applies to all subgoals}}
47 by (Clarify_tac $$i$$);            \emph{\textrm{applies to one subgoal}}
48 \end{ttbox}
51 You need to know how the classical reasoner works in order to use it
52 effectively.  There are many tactics to choose from, including
53 {\tt Fast_tac} and \texttt{Best_tac}.
55 We shall first discuss the underlying principles, then present the
56 classical reasoner.  Finally, we shall see how to instantiate it for new logics.
57 The logics \FOL, \ZF, {\HOL} and {\HOLCF} have it already installed.
60 \section{The sequent calculus}
61 \index{sequent calculus}
62 Isabelle supports natural deduction, which is easy to use for interactive
63 proof.  But natural deduction does not easily lend itself to automation,
64 and has a bias towards intuitionism.  For certain proofs in classical
65 logic, it can not be called natural.  The {\bf sequent calculus}, a
66 generalization of natural deduction, is easier to automate.
68 A {\bf sequent} has the form $\Gamma\turn\Delta$, where $\Gamma$
69 and~$\Delta$ are sets of formulae.%
70 \footnote{For first-order logic, sequents can equivalently be made from
71   lists or multisets of formulae.} The sequent
72 $P@1,\ldots,P@m\turn Q@1,\ldots,Q@n$
73 is {\bf valid} if $P@1\conj\ldots\conj P@m$ implies $Q@1\disj\ldots\disj 74 Q@n$.  Thus $P@1,\ldots,P@m$ represent assumptions, each of which is true,
75 while $Q@1,\ldots,Q@n$ represent alternative goals.  A sequent is {\bf
76 basic} if its left and right sides have a common formula, as in $P,Q\turn 77 Q,R$; basic sequents are trivially valid.
79 Sequent rules are classified as {\bf right} or {\bf left}, indicating which
80 side of the $\turn$~symbol they operate on.  Rules that operate on the
81 right side are analogous to natural deduction's introduction rules, and
82 left rules are analogous to elimination rules.
83 Recall the natural deduction rules for
84   first-order logic,
85 \iflabelundefined{fol-fig}{from {\it Introduction to Isabelle}}%
86                           {Fig.\ts\ref{fol-fig}}.
87 The sequent calculus analogue of~$({\imp}I)$ is the rule
88 $$89 \ainfer{\Gamma &\turn \Delta, P\imp Q}{P,\Gamma &\turn \Delta,Q} 90 \eqno({\imp}R) 91$$
92 This breaks down some implication on the right side of a sequent; $\Gamma$
93 and $\Delta$ stand for the sets of formulae that are unaffected by the
94 inference.  The analogue of the pair~$({\disj}I1)$ and~$({\disj}I2)$ is the
95 single rule
96 $$97 \ainfer{\Gamma &\turn \Delta, P\disj Q}{\Gamma &\turn \Delta,P,Q} 98 \eqno({\disj}R) 99$$
100 This breaks down some disjunction on the right side, replacing it by both
101 disjuncts.  Thus, the sequent calculus is a kind of multiple-conclusion logic.
103 To illustrate the use of multiple formulae on the right, let us prove
104 the classical theorem $(P\imp Q)\disj(Q\imp P)$.  Working backwards, we
105 reduce this formula to a basic sequent:
106 $\infer[(\disj)R]{\turn(P\imp Q)\disj(Q\imp P)} 107 {\infer[(\imp)R]{\turn(P\imp Q), (Q\imp P)\;} 108 {\infer[(\imp)R]{P \turn Q, (Q\imp P)\qquad} 109 {P, Q \turn Q, P\qquad\qquad}}} 110$
111 This example is typical of the sequent calculus: start with the desired
112 theorem and apply rules backwards in a fairly arbitrary manner.  This yields a
113 surprisingly effective proof procedure.  Quantifiers add few complications,
114 since Isabelle handles parameters and schematic variables.  See Chapter~10
115 of {\em ML for the Working Programmer}~\cite{paulson-ml2} for further
116 discussion.
119 \section{Simulating sequents by natural deduction}
120 Isabelle can represent sequents directly, as in the object-logic~\texttt{LK}\@.
121 But natural deduction is easier to work with, and most object-logics employ
122 it.  Fortunately, we can simulate the sequent $P@1,\ldots,P@m\turn 123 Q@1,\ldots,Q@n$ by the Isabelle formula
124 $\List{P@1;\ldots;P@m; \neg Q@2;\ldots; \neg Q@n}\Imp Q@1,$
125 where the order of the assumptions and the choice of~$Q@1$ are arbitrary.
126 Elim-resolution plays a key role in simulating sequent proofs.
128 We can easily handle reasoning on the left.
129 As discussed in
130 \iflabelundefined{destruct}{{\it Introduction to Isabelle}}{\S\ref{destruct}},
131 elim-resolution with the rules $(\disj E)$, $(\bot E)$ and $(\exists E)$
132 achieves a similar effect as the corresponding sequent rules.  For the
133 other connectives, we use sequent-style elimination rules instead of
134 destruction rules such as $({\conj}E1,2)$ and $(\forall E)$.  But note that
135 the rule $(\neg L)$ has no effect under our representation of sequents!
136 $$137 \ainfer{\neg P,\Gamma &\turn \Delta}{\Gamma &\turn \Delta,P}\eqno({\neg}L) 138$$
139 What about reasoning on the right?  Introduction rules can only affect the
140 formula in the conclusion, namely~$Q@1$.  The other right-side formulae are
141 represented as negated assumptions, $\neg Q@2$, \ldots,~$\neg Q@n$.
142 \index{assumptions!negated}
143 In order to operate on one of these, it must first be exchanged with~$Q@1$.
144 Elim-resolution with the {\bf swap} rule has this effect:
145 $$\List{\neg P; \; \neg R\Imp P} \Imp R \eqno(swap)$$
146 To ensure that swaps occur only when necessary, each introduction rule is
147 converted into a swapped form: it is resolved with the second premise
148 of~$(swap)$.  The swapped form of~$({\conj}I)$, which might be
149 called~$({\neg\conj}E)$, is
150 $\List{\neg(P\conj Q); \; \neg R\Imp P; \; \neg R\Imp Q} \Imp R.$
151 Similarly, the swapped form of~$({\imp}I)$ is
152 $\List{\neg(P\imp Q); \; \List{\neg R;P}\Imp Q} \Imp R$
153 Swapped introduction rules are applied using elim-resolution, which deletes
154 the negated formula.  Our representation of sequents also requires the use
155 of ordinary introduction rules.  If we had no regard for readability, we
156 could treat the right side more uniformly by representing sequents as
157 $\List{P@1;\ldots;P@m; \neg Q@1;\ldots; \neg Q@n}\Imp \bot.$
160 \section{Extra rules for the sequent calculus}
161 As mentioned, destruction rules such as $({\conj}E1,2)$ and $(\forall E)$
162 must be replaced by sequent-style elimination rules.  In addition, we need
163 rules to embody the classical equivalence between $P\imp Q$ and $\neg P\disj 164 Q$.  The introduction rules~$({\disj}I1,2)$ are replaced by a rule that
165 simulates $({\disj}R)$:
166 $(\neg Q\Imp P) \Imp P\disj Q$
167 The destruction rule $({\imp}E)$ is replaced by
168 $\List{P\imp Q;\; \neg P\Imp R;\; Q\Imp R} \Imp R.$
169 Quantifier replication also requires special rules.  In classical logic,
170 $\exists x{.}P$ is equivalent to $\neg\forall x{.}\neg P$; the rules
171 $(\exists R)$ and $(\forall L)$ are dual:
172 $\ainfer{\Gamma &\turn \Delta, \exists x{.}P} 173 {\Gamma &\turn \Delta, \exists x{.}P, P[t/x]} \; (\exists R) 174 \qquad 175 \ainfer{\forall x{.}P, \Gamma &\turn \Delta} 176 {P[t/x], \forall x{.}P, \Gamma &\turn \Delta} \; (\forall L) 177$
178 Thus both kinds of quantifier may be replicated.  Theorems requiring
179 multiple uses of a universal formula are easy to invent; consider
180 $(\forall x.P(x)\imp P(f(x))) \conj P(a) \imp P(f^n(a)),$
181 for any~$n>1$.  Natural examples of the multiple use of an existential
182 formula are rare; a standard one is $\exists x.\forall y. P(x)\imp P(y)$.
184 Forgoing quantifier replication loses completeness, but gains decidability,
185 since the search space becomes finite.  Many useful theorems can be proved
186 without replication, and the search generally delivers its verdict in a
187 reasonable time.  To adopt this approach, represent the sequent rules
188 $(\exists R)$, $(\exists L)$ and $(\forall R)$ by $(\exists I)$, $(\exists 189 E)$ and $(\forall I)$, respectively, and put $(\forall E)$ into elimination
190 form:
191 $$\List{\forall x{.}P(x); P(t)\Imp Q} \Imp Q \eqno(\forall E@2)$$
192 Elim-resolution with this rule will delete the universal formula after a
193 single use.  To replicate universal quantifiers, replace the rule by
194 $$195 \List{\forall x{.}P(x);\; \List{P(t); \forall x{.}P(x)}\Imp Q} \Imp Q. 196 \eqno(\forall E@3) 197$$
198 To replicate existential quantifiers, replace $(\exists I)$ by
199 $\List{\neg(\exists x{.}P(x)) \Imp P(t)} \Imp \exists x{.}P(x).$
200 All introduction rules mentioned above are also useful in swapped form.
202 Replication makes the search space infinite; we must apply the rules with
203 care.  The classical reasoner distinguishes between safe and unsafe
204 rules, applying the latter only when there is no alternative.  Depth-first
205 search may well go down a blind alley; best-first search is better behaved
206 in an infinite search space.  However, quantifier replication is too
207 expensive to prove any but the simplest theorems.
210 \section{Classical rule sets}
211 \index{classical sets}
212 Each automatic tactic takes a {\bf classical set} --- a collection of
213 rules, classified as introduction or elimination and as {\bf safe} or {\bf
214 unsafe}.  In general, safe rules can be attempted blindly, while unsafe
215 rules must be used with care.  A safe rule must never reduce a provable
216 goal to an unprovable set of subgoals.
218 The rule~$({\disj}I1)$ is unsafe because it reduces $P\disj Q$ to~$P$.  Any
219 rule is unsafe whose premises contain new unknowns.  The elimination
220 rule~$(\forall E@2)$ is unsafe, since it is applied via elim-resolution,
221 which discards the assumption $\forall x{.}P(x)$ and replaces it by the
222 weaker assumption~$P(\Var{t})$.  The rule $({\exists}I)$ is unsafe for
223 similar reasons.  The rule~$(\forall E@3)$ is unsafe in a different sense:
224 since it keeps the assumption $\forall x{.}P(x)$, it is prone to looping.
225 In classical first-order logic, all rules are safe except those mentioned
226 above.
228 The safe/unsafe distinction is vague, and may be regarded merely as a way
229 of giving some rules priority over others.  One could argue that
230 $({\disj}E)$ is unsafe, because repeated application of it could generate
231 exponentially many subgoals.  Induction rules are unsafe because inductive
232 proofs are difficult to set up automatically.  Any inference is unsafe that
233 instantiates an unknown in the proof state --- thus \ttindex{match_tac}
234 must be used, rather than \ttindex{resolve_tac}.  Even proof by assumption
235 is unsafe if it instantiates unknowns shared with other subgoals --- thus
236 \ttindex{eq_assume_tac} must be used, rather than \ttindex{assume_tac}.
238 \subsection{Adding rules to classical sets}
239 Classical rule sets belong to the abstract type \mltydx{claset}, which
240 supports the following operations (provided the classical reasoner is
241 installed!):
242 \begin{ttbox}
243 empty_cs : claset
244 print_cs : claset -> unit
245 rep_cs : claset -> \{safeEs: thm list, safeIs: thm list,
246                     hazEs: thm list,  hazIs: thm list,
247                     swrappers: (string * wrapper) list,
248                     uwrappers: (string * wrapper) list,
249                     safe0_netpair: netpair, safep_netpair: netpair,
250                     haz_netpair: netpair, dup_netpair: netpair\}
251 addSIs   : claset * thm list -> claset                 \hfill{\bf infix 4}
252 addSEs   : claset * thm list -> claset                 \hfill{\bf infix 4}
253 addSDs   : claset * thm list -> claset                 \hfill{\bf infix 4}
254 addIs    : claset * thm list -> claset                 \hfill{\bf infix 4}
255 addEs    : claset * thm list -> claset                 \hfill{\bf infix 4}
256 addDs    : claset * thm list -> claset                 \hfill{\bf infix 4}
257 delrules : claset * thm list -> claset                 \hfill{\bf infix 4}
258 \end{ttbox}
259 The add operations ignore any rule already present in the claset with the same
260 classification (such as safe introduction).  They print a warning if the rule
261 has already been added with some other classification, but add the rule
262 anyway.  Calling \texttt{delrules} deletes all occurrences of a rule from the
263 claset, but see the warning below concerning destruction rules.
264 \begin{ttdescription}
265 \item[\ttindexbold{empty_cs}] is the empty classical set.
267 \item[\ttindexbold{print_cs} $cs$] displays the printable contents of~$cs$,
268   which is the rules. All other parts are non-printable.
270 \item[\ttindexbold{rep_cs} $cs$] decomposes $cs$ as a record of its internal
271   components, namely the safe introduction and elimination rules, the unsafe
272   introduction and elimination rules, the lists of safe and unsafe wrappers
273   (see \ref{sec:modifying-search}), and the internalized forms of the rules.
275 \item[$cs$ addSIs $rules$] \indexbold{*addSIs}
276 adds safe introduction~$rules$ to~$cs$.
278 \item[$cs$ addSEs $rules$] \indexbold{*addSEs}
279 adds safe elimination~$rules$ to~$cs$.
281 \item[$cs$ addSDs $rules$] \indexbold{*addSDs}
282 adds safe destruction~$rules$ to~$cs$.
284 \item[$cs$ addIs $rules$] \indexbold{*addIs}
285 adds unsafe introduction~$rules$ to~$cs$.
287 \item[$cs$ addEs $rules$] \indexbold{*addEs}
288 adds unsafe elimination~$rules$ to~$cs$.
290 \item[$cs$ addDs $rules$] \indexbold{*addDs}
291 adds unsafe destruction~$rules$ to~$cs$.
293 \item[$cs$ delrules $rules$] \indexbold{*delrules}
294 deletes~$rules$ from~$cs$.  It prints a warning for those rules that are not
295 in~$cs$.
296 \end{ttdescription}
298 \begin{warn}
299   If you added $rule$ using \texttt{addSDs} or \texttt{addDs}, then you must delete
300   it as follows:
301 \begin{ttbox}
302 $$cs$$ delrules [make_elim $$rule$$]
303 \end{ttbox}
304 \par\noindent
305 This is necessary because the operators \texttt{addSDs} and \texttt{addDs} convert
306 the destruction rules to elimination rules by applying \ttindex{make_elim},
307 and then insert them using \texttt{addSEs} and \texttt{addEs}, respectively.
308 \end{warn}
310 Introduction rules are those that can be applied using ordinary resolution.
311 The classical set automatically generates their swapped forms, which will
312 be applied using elim-resolution.  Elimination rules are applied using
313 elim-resolution.  In a classical set, rules are sorted by the number of new
314 subgoals they will yield; rules that generate the fewest subgoals will be
315 tried first (see \S\ref{biresolve_tac}).
317 For elimination and destruction rules there are variants of the add operations
318 adding a rule in a way such that it is applied only if also its second premise
319 can be unified with an assumption of the current proof state:
321 \begin{ttbox}
322 addSE2      : claset * (string * thm) -> claset           \hfill{\bf infix 4}
323 addSD2      : claset * (string * thm) -> claset           \hfill{\bf infix 4}
324 addE2       : claset * (string * thm) -> claset           \hfill{\bf infix 4}
325 addD2       : claset * (string * thm) -> claset           \hfill{\bf infix 4}
326 \end{ttbox}
327 \begin{warn}
328   A rule to be added in this special way must be given a name, which is used
329   to delete it again -- when desired -- using \texttt{delSWrappers} or
330   \texttt{delWrappers}, respectively. This is because these add operations
331   are implemented as wrappers (see \ref{sec:modifying-search} below).
332 \end{warn}
335 \subsection{Modifying the search step}
336 \label{sec:modifying-search}
337 For a given classical set, the proof strategy is simple.  Perform as many safe
338 inferences as possible; or else, apply certain safe rules, allowing
339 instantiation of unknowns; or else, apply an unsafe rule.  The tactics also
340 eliminate assumptions of the form $x=t$ by substitution if they have been set
341 up to do so (see \texttt{hyp_subst_tacs} in~\S\ref{sec:classical-setup} below).
342 They may perform a form of Modus Ponens: if there are assumptions $P\imp Q$
343 and~$P$, then replace $P\imp Q$ by~$Q$.
345 The classical reasoning tactics --- except \texttt{blast_tac}! --- allow
346 you to modify this basic proof strategy by applying two lists of arbitrary
347 {\bf wrapper tacticals} to it.
348 The first wrapper list, which is considered to contain safe wrappers only,
349 affects \ttindex{safe_step_tac} and all the tactics that call it.
350 The second one, which may contain unsafe wrappers, affects the unsafe parts
351 of \ttindex{step_tac}, \ttindex{slow_step_tac}, and the tactics that call them.
352 A wrapper transforms each step of the search, for example
353 by attempting other tactics before or after the original step tactic.
354 All members of a wrapper list are applied in turn to the respective step tactic.
356 Initially the two wrapper lists are empty, which means no modification of the
357 step tactics. Safe and unsafe wrappers are added to a claset
358 with the functions given below, supplying them with wrapper names.
359 These names may be used to selectively delete wrappers.
361 \begin{ttbox}
362 type wrapper = (int -> tactic) -> (int -> tactic);
364 addSWrapper  : claset * (string *  wrapper       ) -> claset \hfill{\bf infix 4}
365 addSbefore   : claset * (string * (int -> tactic)) -> claset \hfill{\bf infix 4}
366 addSaltern   : claset * (string * (int -> tactic)) -> claset \hfill{\bf infix 4}
367 delSWrapper  : claset *  string                    -> claset \hfill{\bf infix 4}
369 addWrapper   : claset * (string *  wrapper       ) -> claset \hfill{\bf infix 4}
370 addbefore    : claset * (string * (int -> tactic)) -> claset \hfill{\bf infix 4}
371 addaltern    : claset * (string * (int -> tactic)) -> claset \hfill{\bf infix 4}
372 delWrapper   : claset *  string                    -> claset \hfill{\bf infix 4}
374 addSss       : claset * simpset -> claset                 \hfill{\bf infix 4}
375 addss        : claset * simpset -> claset                 \hfill{\bf infix 4}
376 \end{ttbox}
377 %
379 \begin{ttdescription}
380 \item[$cs$ addSWrapper $(name,wrapper)$] \indexbold{*addSWrapper}
381 adds a new wrapper, which should yield a safe tactic,
382 to modify the existing safe step tactic.
384 \item[$cs$ addSbefore $(name,tac)$] \indexbold{*addSbefore}
385 adds the given tactic as a safe wrapper, such that it is tried
386 {\em before} each safe step of the search.
388 \item[$cs$ addSaltern $(name,tac)$] \indexbold{*addSaltern}
389 adds the given tactic as a safe wrapper, such that it is tried
390 when a safe step of the search would fail.
392 \item[$cs$ delSWrapper $name$] \indexbold{*delSWrapper}
393 deletes the safe wrapper with the given name.
395 \item[$cs$ addWrapper $(name,wrapper)$] \indexbold{*addWrapper}
396 adds a new wrapper to modify the existing (unsafe) step tactic.
398 \item[$cs$ addbefore $(name,tac)$] \indexbold{*addbefore}
399 adds the given tactic as an unsafe wrapper, such that it its result is
400 concatenated {\em before} the result of each unsafe step.
402 \item[$cs$ addaltern $(name,tac)$] \indexbold{*addaltern}
403 adds the given tactic as an unsafe wrapper, such that it its result is
404 concatenated {\em after} the result of each unsafe step.
406 \item[$cs$ delWrapper $name$] \indexbold{*delWrapper}
407 deletes the unsafe wrapper with the given name.
409 \item[$cs$ addSss $ss$] \indexbold{*addss}
410 adds the simpset~$ss$ to the classical set.  The assumptions and goal will be
411 simplified, in a rather safe way, after each safe step of the search.
413 \item[$cs$ addss $ss$] \indexbold{*addss}
414 adds the simpset~$ss$ to the classical set.  The assumptions and goal will be
415 simplified, before the each unsafe step of the search.
417 \end{ttdescription}
419 \index{simplification!from classical reasoner}
420 Strictly speaking, the operators \texttt{addss} and \texttt{addSss}
421 are not part of the classical reasoner.
422 , which are used as primitives
423 for the automatic tactics described in \S\ref{sec:automatic-tactics}, are
424 implemented as wrapper tacticals.
425 they
426 \begin{warn}
427 Being defined as wrappers, these operators are inappropriate for adding more
428 than one simpset at a time: the simpset added last overwrites any earlier ones.
429 When a simpset combined with a claset is to be augmented, this should done
430 {\em before} combining it with the claset.
431 \end{warn}
434 \section{The classical tactics}
435 \index{classical reasoner!tactics} If installed, the classical module provides
436 powerful theorem-proving tactics.  Most of them have capitalized analogues
437 that use the default claset; see \S\ref{sec:current-claset}.
440 \subsection{The tableau prover}
441 The tactic \texttt{blast_tac} searches for a proof using a fast tableau prover,
442 coded directly in \ML.  It then reconstructs the proof using Isabelle
443 tactics.  It is faster and more powerful than the other classical
444 reasoning tactics, but has major limitations too.
445 \begin{itemize}
446 \item It does not use the wrapper tacticals described above, such as
448 \item It ignores types, which can cause problems in \HOL.  If it applies a rule
449   whose types are inappropriate, then proof reconstruction will fail.
450 \item It does not perform higher-order unification, as needed by the rule {\tt
451     rangeI} in {\HOL} and \texttt{RepFunI} in {\ZF}.  There are often
452     alternatives to such rules, for example {\tt
453     range_eqI} and \texttt{RepFun_eqI}.
454 \item Function variables may only be applied to parameters of the subgoal.
455 (This restriction arises because the prover does not use higher-order
456 unification.)  If other function variables are present then the prover will
457 fail with the message {\small\tt Function Var's argument not a bound variable}.
458 \item Its proof strategy is more general than \texttt{fast_tac}'s but can be
459   slower.  If \texttt{blast_tac} fails or seems to be running forever, try {\tt
460   fast_tac} and the other tactics described below.
461 \end{itemize}
462 %
463 \begin{ttbox}
464 blast_tac        : claset -> int -> tactic
465 Blast.depth_tac  : claset -> int -> int -> tactic
466 Blast.trace      : bool ref \hfill{\bf initially false}
467 \end{ttbox}
468 The two tactics differ on how they bound the number of unsafe steps used in a
469 proof.  While \texttt{blast_tac} starts with a bound of zero and increases it
470 successively to~20, \texttt{Blast.depth_tac} applies a user-supplied search bound.
471 \begin{ttdescription}
472 \item[\ttindexbold{blast_tac} $cs$ $i$] tries to prove
473   subgoal~$i$, increasing the search bound using iterative
474   deepening~\cite{korf85}.
476 \item[\ttindexbold{Blast.depth_tac} $cs$ $lim$ $i$] tries
477   to prove subgoal~$i$ using a search bound of $lim$.  Sometimes a slow
478   proof using \texttt{blast_tac} can be made much faster by supplying the
479   successful search bound to this tactic instead.
481 \item[set \ttindexbold{Blast.trace};] \index{tracing!of classical prover}
482   causes the tableau prover to print a trace of its search.  At each step it
483   displays the formula currently being examined and reports whether the branch
484   has been closed, extended or split.
485 \end{ttdescription}
488 \subsection{Automatic tactics}\label{sec:automatic-tactics}
489 \begin{ttbox}
490 type clasimpset = claset * simpset;
491 auto_tac        : clasimpset ->        tactic
492 force_tac       : clasimpset -> int -> tactic
493 auto            : unit -> unit
494 force           : int  -> unit
495 \end{ttbox}
496 The automatic tactics attempt to prove goals using a combination of
497 simplification and classical reasoning.
498 \begin{ttdescription}
499 \item[\ttindexbold{auto_tac $(cs,ss)$}] is intended for situations where
500 there are a lot of mostly trivial subgoals; it proves all the easy ones,
501 leaving the ones it cannot prove.
502 (Unfortunately, attempting to prove the hard ones may take a long time.)
503 \item[\ttindexbold{force_tac} $(cs,ss)$ $i$] is intended to prove subgoal~$i$
504 completely. It tries to apply all fancy tactics it knows about,
505 performing a rather exhaustive search.
506 \end{ttdescription}
507 They must be supplied both a simpset and a claset; therefore
508 they are most easily called as \texttt{Auto_tac} and \texttt{Force_tac}, which
509 use the default claset and simpset (see \S\ref{sec:current-claset} below).
510 For interactive use,
511 the shorthand \texttt{auto();} abbreviates \texttt{by Auto_tac;}
512 while \texttt{force 1;} abbreviates \texttt{by (Force_tac 1);}
515 \subsection{Semi-automatic tactics}
516 \begin{ttbox}
517 clarify_tac      : claset -> int -> tactic
518 clarify_step_tac : claset -> int -> tactic
519 clarsimp_tac     : clasimpset -> int -> tactic
520 \end{ttbox}
521 Use these when the automatic tactics fail.  They perform all the obvious
522 logical inferences that do not split the subgoal.  The result is a
523 simpler subgoal that can be tackled by other means, such as by
524 instantiating quantifiers yourself.
525 \begin{ttdescription}
526 \item[\ttindexbold{clarify_tac} $cs$ $i$] performs a series of safe steps on
527 subgoal~$i$ by repeatedly calling \texttt{clarify_step_tac}.
528 \item[\ttindexbold{clarify_step_tac} $cs$ $i$] performs a safe step on
529   subgoal~$i$.  No splitting step is applied; for example, the subgoal $A\conj 530 B$ is left as a conjunction.  Proof by assumption, Modus Ponens, etc., may be
531   performed provided they do not instantiate unknowns.  Assumptions of the
532   form $x=t$ may be eliminated.  The user-supplied safe wrapper tactical is
533   applied.
534 \item[\ttindexbold{clarsimp_tac} $cs$ $i$] acts like \texttt{clarify_tac}, but
535 also does simplification with the given simpset. note that if the simpset
536 includes a splitter for the premises, the subgoal may still be split.
537 \end{ttdescription}
540 \subsection{Other classical tactics}
541 \begin{ttbox}
542 fast_tac      : claset -> int -> tactic
543 best_tac      : claset -> int -> tactic
544 slow_tac      : claset -> int -> tactic
545 slow_best_tac : claset -> int -> tactic
546 \end{ttbox}
547 These tactics attempt to prove a subgoal using sequent-style reasoning.
548 Unlike \texttt{blast_tac}, they construct proofs directly in Isabelle.  Their
549 effect is restricted (by \texttt{SELECT_GOAL}) to one subgoal; they either prove
550 this subgoal or fail.  The \texttt{slow_} versions conduct a broader
551 search.%
552 \footnote{They may, when backtracking from a failed proof attempt, undo even
553   the step of proving a subgoal by assumption.}
555 The best-first tactics are guided by a heuristic function: typically, the
556 total size of the proof state.  This function is supplied in the functor call
557 that sets up the classical reasoner.
558 \begin{ttdescription}
559 \item[\ttindexbold{fast_tac} $cs$ $i$] applies \texttt{step_tac} using
560 depth-first search to prove subgoal~$i$.
562 \item[\ttindexbold{best_tac} $cs$ $i$] applies \texttt{step_tac} using
563 best-first search to prove subgoal~$i$.
565 \item[\ttindexbold{slow_tac} $cs$ $i$] applies \texttt{slow_step_tac} using
566 depth-first search to prove subgoal~$i$.
568 \item[\ttindexbold{slow_best_tac} $cs$ $i$] applies \texttt{slow_step_tac} with
569 best-first search to prove subgoal~$i$.
570 \end{ttdescription}
573 \subsection{Depth-limited automatic tactics}
574 \begin{ttbox}
575 depth_tac  : claset -> int -> int -> tactic
576 deepen_tac : claset -> int -> int -> tactic
577 \end{ttbox}
578 These work by exhaustive search up to a specified depth.  Unsafe rules are
579 modified to preserve the formula they act on, so that it be used repeatedly.
580 They can prove more goals than \texttt{fast_tac} can but are much
581 slower, for example if the assumptions have many universal quantifiers.
583 The depth limits the number of unsafe steps.  If you can estimate the minimum
584 number of unsafe steps needed, supply this value as~$m$ to save time.
585 \begin{ttdescription}
586 \item[\ttindexbold{depth_tac} $cs$ $m$ $i$]
587 tries to prove subgoal~$i$ by exhaustive search up to depth~$m$.
589 \item[\ttindexbold{deepen_tac} $cs$ $m$ $i$]
590 tries to prove subgoal~$i$ by iterative deepening.  It calls \texttt{depth_tac}
591 repeatedly with increasing depths, starting with~$m$.
592 \end{ttdescription}
595 \subsection{Single-step tactics}
596 \begin{ttbox}
597 safe_step_tac : claset -> int -> tactic
598 safe_tac      : claset        -> tactic
599 inst_step_tac : claset -> int -> tactic
600 step_tac      : claset -> int -> tactic
601 slow_step_tac : claset -> int -> tactic
602 \end{ttbox}
603 The automatic proof procedures call these tactics.  By calling them
604 yourself, you can execute these procedures one step at a time.
605 \begin{ttdescription}
606 \item[\ttindexbold{safe_step_tac} $cs$ $i$] performs a safe step on
607   subgoal~$i$.  The safe wrapper tacticals are applied to a tactic that may
608   include proof by assumption or Modus Ponens (taking care not to instantiate
609   unknowns), or substitution.
611 \item[\ttindexbold{safe_tac} $cs$] repeatedly performs safe steps on all
612 subgoals.  It is deterministic, with at most one outcome.
614 \item[\ttindexbold{inst_step_tac} $cs$ $i$] is like \texttt{safe_step_tac},
615 but allows unknowns to be instantiated.
617 \item[\ttindexbold{step_tac} $cs$ $i$] is the basic step of the proof
618   procedure.  The unsafe wrapper tacticals are applied to a tactic that tries
619   \texttt{safe_tac}, \texttt{inst_step_tac}, or applies an unsafe rule
620   from~$cs$.
622 \item[\ttindexbold{slow_step_tac}]
623   resembles \texttt{step_tac}, but allows backtracking between using safe
624   rules with instantiation (\texttt{inst_step_tac}) and using unsafe rules.
625   The resulting search space is larger.
626 \end{ttdescription}
629 \subsection{The current claset}\label{sec:current-claset}
631 Each theory is equipped with an implicit \emph{current claset}
632 \index{claset!current}.  This is a default set of classical
633 rules.  The underlying idea is quite similar to that of a current
634 simpset described in \S\ref{sec:simp-for-dummies}; please read that
635 section, including its warnings.
637 The tactics
638 \begin{ttbox}
639 Blast_tac        : int -> tactic
640 Auto_tac         :        tactic
641 Force_tac        : int -> tactic
642 Fast_tac         : int -> tactic
643 Best_tac         : int -> tactic
644 Deepen_tac       : int -> int -> tactic
645 Clarify_tac      : int -> tactic
646 Clarify_step_tac : int -> tactic
647 Clarsimp_tac     : int -> tactic
648 Safe_tac         :        tactic
649 Safe_step_tac    : int -> tactic
650 Step_tac         : int -> tactic
651 \end{ttbox}
652 \indexbold{*Blast_tac}\indexbold{*Auto_tac}\indexbold{*Force_tac}
653 \indexbold{*Best_tac}\indexbold{*Fast_tac}%
654 \indexbold{*Deepen_tac}
655 \indexbold{*Clarify_tac}\indexbold{*Clarify_step_tac}\indexbold{*Clarsimp_tac}
656 \indexbold{*Safe_tac}\indexbold{*Safe_step_tac}
657 \indexbold{*Step_tac}
658 make use of the current claset.  For example, \texttt{Blast_tac} is defined as
659 \begin{ttbox}
660 fun Blast_tac i st = blast_tac (claset()) i st;
661 \end{ttbox}
662 and gets the current claset, only after it is applied to a proof state.
663 The functions
664 \begin{ttbox}
666 \end{ttbox}
669 are used to add rules to the current claset.  They work exactly like their
670 lower case counterparts, such as \texttt{addSIs}.  Calling
671 \begin{ttbox}
672 Delrules : thm list -> unit
673 \end{ttbox}
674 deletes rules from the current claset.
676 \medskip A few further functions are available as uppercase versions only:
677 \begin{ttbox}
679 \end{ttbox}
681 current claset by \emph{extra} introduction, elimination, or destruct rules.
682 These provide additional hints for the basic non-automated proof methods of
683 Isabelle/Isar \cite{isabelle-isar-ref}.  The corresponding Isar attributes are
684 $intro?$'', $elim?$'', and $dest?$''.  Note that these extra rules do
685 not have any effect on classic Isabelle tactics.
688 \subsection{Accessing the current claset}
689 \label{sec:access-current-claset}
691 the functions to access the current claset are analogous to the functions
692 for the current simpset, so please see \ref{sec:access-current-simpset}
693 for a description.
694 \begin{ttbox}
695 claset        : unit   -> claset
696 claset_ref    : unit   -> claset ref
697 claset_of     : theory -> claset
698 claset_ref_of : theory -> claset ref
699 print_claset  : theory -> unit
700 CLASET        :(claset     ->       tactic) ->       tactic
701 CLASET'       :(claset     -> 'a -> tactic) -> 'a -> tactic
702 CLASIMPSET    :(clasimpset ->       tactic) ->       tactic
703 CLASIMPSET'   :(clasimpset -> 'a -> tactic) -> 'a -> tactic
704 \end{ttbox}
707 \subsection{Other useful tactics}
709 \index{tactics!for Modus Ponens}
710 \begin{ttbox}
711 contr_tac    :             int -> tactic
712 mp_tac       :             int -> tactic
713 eq_mp_tac    :             int -> tactic
714 swap_res_tac : thm list -> int -> tactic
715 \end{ttbox}
716 These can be used in the body of a specialized search.
717 \begin{ttdescription}
718 \item[\ttindexbold{contr_tac} {\it i}]\index{assumptions!contradictory}
719   solves subgoal~$i$ by detecting a contradiction among two assumptions of
720   the form $P$ and~$\neg P$, or fail.  It may instantiate unknowns.  The
721   tactic can produce multiple outcomes, enumerating all possible
724 \item[\ttindexbold{mp_tac} {\it i}]
725 is like \texttt{contr_tac}, but also attempts to perform Modus Ponens in
726 subgoal~$i$.  If there are assumptions $P\imp Q$ and~$P$, then it replaces
727 $P\imp Q$ by~$Q$.  It may instantiate unknowns.  It fails if it can do
728 nothing.
730 \item[\ttindexbold{eq_mp_tac} {\it i}]
731 is like \texttt{mp_tac} {\it i}, but may not instantiate unknowns --- thus, it
732 is safe.
734 \item[\ttindexbold{swap_res_tac} {\it thms} {\it i}] refines subgoal~$i$ of
735 the proof state using {\it thms}, which should be a list of introduction
736 rules.  First, it attempts to prove the goal using \texttt{assume_tac} or
737 \texttt{contr_tac}.  It then attempts to apply each rule in turn, attempting
738 resolution and also elim-resolution with the swapped form.
739 \end{ttdescription}
741 \subsection{Creating swapped rules}
742 \begin{ttbox}
743 swapify   : thm list -> thm list
744 joinrules : thm list * thm list -> (bool * thm) list
745 \end{ttbox}
746 \begin{ttdescription}
747 \item[\ttindexbold{swapify} {\it thms}] returns a list consisting of the
748 swapped versions of~{\it thms}, regarded as introduction rules.
750 \item[\ttindexbold{joinrules} ({\it intrs}, {\it elims})]
751 joins introduction rules, their swapped versions, and elimination rules for
752 use with \ttindex{biresolve_tac}.  Each rule is paired with~\texttt{false}
753 (indicating ordinary resolution) or~\texttt{true} (indicating
754 elim-resolution).
755 \end{ttdescription}
758 \section{Setting up the classical reasoner}\label{sec:classical-setup}
759 \index{classical reasoner!setting up}
760 Isabelle's classical object-logics, including \texttt{FOL} and \texttt{HOL},
761 have the classical reasoner already set up.
762 When defining a new classical logic, you should set up the reasoner yourself.
763 It consists of the \ML{} functor \ttindex{ClassicalFun}, which takes the
764 argument signature \texttt{CLASSICAL_DATA}:
765 \begin{ttbox}
766 signature CLASSICAL_DATA =
767   sig
768   val mp             : thm
769   val not_elim       : thm
770   val swap           : thm
771   val sizef          : thm -> int
772   val hyp_subst_tacs : (int -> tactic) list
773   end;
774 \end{ttbox}
775 Thus, the functor requires the following items:
776 \begin{ttdescription}
777 \item[\tdxbold{mp}] should be the Modus Ponens rule
778 $\List{\Var{P}\imp\Var{Q};\; \Var{P}} \Imp \Var{Q}$.
780 \item[\tdxbold{not_elim}] should be the contradiction rule
781 $\List{\neg\Var{P};\; \Var{P}} \Imp \Var{R}$.
783 \item[\tdxbold{swap}] should be the swap rule
784 $\List{\neg \Var{P}; \; \neg \Var{R}\Imp \Var{P}} \Imp \Var{R}$.
786 \item[\ttindexbold{sizef}] is the heuristic function used for best-first
787 search.  It should estimate the size of the remaining subgoals.  A good
788 heuristic function is \ttindex{size_of_thm}, which measures the size of the
789 proof state.  Another size function might ignore certain subgoals (say,
790 those concerned with type-checking).  A heuristic function might simply
791 count the subgoals.
793 \item[\ttindexbold{hyp_subst_tacs}] is a list of tactics for substitution in
794 the hypotheses, typically created by \ttindex{HypsubstFun} (see
795 Chapter~\ref{substitution}).  This list can, of course, be empty.  The
796 tactics are assumed to be safe!
797 \end{ttdescription}
798 The functor is not at all sensitive to the formalization of the
799 object-logic.  It does not even examine the rules, but merely applies
800 them according to its fixed strategy.  The functor resides in {\tt
801   Provers/classical.ML} in the Isabelle sources.
803 \index{classical reasoner|)}
805 \section{Setting up the combination with the simplifier}
806 \label{sec:clasimp-setup}
808 To combine the classical reasoner and the simplifier, we simply call the
809 \ML{} functor \ttindex{ClasimpFun} that assembles the parts as required.
810 It takes a structure (of signature \texttt{CLASIMP_DATA}) as
811 argment, which can be contructed on the fly:
812 \begin{ttbox}
813 structure Clasimp = ClasimpFun
814  (structure Simplifier = Simplifier
815         and Classical  = Classical
816         and Blast      = Blast);
817 \end{ttbox}
818 %
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