src/FOLP/classical.ML
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(*  Title:      FOLP/classical
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    ID:         $Id$
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1992  University of Cambridge
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Like Provers/classical but modified because match_tac is unsuitable for
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proof objects.
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Theorem prover for classical reasoning, including predicate calculus, set
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theory, etc.
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Rules must be classified as intr, elim, safe, hazardous.
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A rule is unsafe unless it can be applied blindly without harmful results.
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For a rule to be safe, its premises and conclusion should be logically
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equivalent.  There should be no variables in the premises that are not in
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the conclusion.
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*)
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signature CLASSICAL_DATA =
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  sig
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  val mp: thm                   (* [| P-->Q;  P |] ==> Q *)
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  val not_elim: thm             (* [| ~P;  P |] ==> R *)
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  val swap: thm                 (* ~P ==> (~Q ==> P) ==> Q *)
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  val sizef : thm -> int        (* size function for BEST_FIRST *)
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  val hyp_subst_tacs: (int -> tactic) list
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  end;
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(*Higher precedence than := facilitates use of references*)
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infix 4 addSIs addSEs addSDs addIs addEs addDs;
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signature CLASSICAL =
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  sig
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  type claset
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  val empty_cs: claset
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  val addDs : claset * thm list -> claset
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  val addEs : claset * thm list -> claset
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  val addIs : claset * thm list -> claset
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  val addSDs: claset * thm list -> claset
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  val addSEs: claset * thm list -> claset
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  val addSIs: claset * thm list -> claset
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  val print_cs: claset -> unit
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  val rep_cs: claset -> 
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      {safeIs: thm list, safeEs: thm list, hazIs: thm list, hazEs: thm list, 
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       safe0_brls:(bool*thm)list, safep_brls: (bool*thm)list,
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       haz_brls: (bool*thm)list}
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  val best_tac : claset -> int -> tactic
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  val contr_tac : int -> tactic
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  val fast_tac : claset -> int -> tactic
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  val inst_step_tac : int -> tactic
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  val joinrules : thm list * thm list -> (bool * thm) list
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  val mp_tac: int -> tactic
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  val safe_tac : claset -> tactic
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  val safe_step_tac : claset -> int -> tactic
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  val slow_step_tac : claset -> int -> tactic
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  val step_tac : claset -> int -> tactic
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  val swapify : thm list -> thm list
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  val swap_res_tac : thm list -> int -> tactic
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  val uniq_mp_tac: int -> tactic
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  end;
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functor ClassicalFun(Data: CLASSICAL_DATA): CLASSICAL = 
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struct
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local open Data in
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(** Useful tactics for classical reasoning **)
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val imp_elim = make_elim mp;
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(*Solve goal that assumes both P and ~P. *)
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val contr_tac = etac not_elim THEN'  assume_tac;
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(*Finds P-->Q and P in the assumptions, replaces implication by Q *)
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fun mp_tac i = eresolve_tac ([not_elim,imp_elim]) i  THEN  assume_tac i;
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(*Like mp_tac but instantiates no variables*)
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fun uniq_mp_tac i = ematch_tac ([not_elim,imp_elim]) i  THEN  uniq_assume_tac i;
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(*Creates rules to eliminate ~A, from rules to introduce A*)
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fun swapify intrs = intrs RLN (2, [swap]);
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(*Uses introduction rules in the normal way, or on negated assumptions,
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  trying rules in order. *)
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fun swap_res_tac rls = 
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    let fun tacf rl = rtac rl ORELSE' etac (rl RSN (2,swap))
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    in  assume_tac ORELSE' contr_tac ORELSE' FIRST' (map tacf rls)
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    end;
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(*** Classical rule sets ***)
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datatype claset =
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 CS of {safeIs: thm list,
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        safeEs: thm list,
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        hazIs: thm list,
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        hazEs: thm list,
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        (*the following are computed from the above*)
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        safe0_brls: (bool*thm)list,
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        safep_brls: (bool*thm)list,
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        haz_brls: (bool*thm)list};
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fun rep_cs (CS x) = x;
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(*For use with biresolve_tac.  Combines intrs with swap to catch negated
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  assumptions.  Also pairs elims with true. *)
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fun joinrules (intrs,elims) =  
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  map (pair true) (elims @ swapify intrs)  @  map (pair false) intrs;
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(*Note that allE precedes exI in haz_brls*)
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fun make_cs {safeIs,safeEs,hazIs,hazEs} =
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  let val (safe0_brls, safep_brls) = (*0 subgoals vs 1 or more*)
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          partition (apl(0,op=) o subgoals_of_brl) 
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             (sort (make_ord lessb) (joinrules(safeIs, safeEs)))
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  in CS{safeIs=safeIs, safeEs=safeEs, hazIs=hazIs, hazEs=hazEs,
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        safe0_brls=safe0_brls, safep_brls=safep_brls,
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        haz_brls = sort (make_ord lessb) (joinrules(hazIs, hazEs))}
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  end;
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(*** Manipulation of clasets ***)
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val empty_cs = make_cs{safeIs=[], safeEs=[], hazIs=[], hazEs=[]};
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fun print_cs (CS{safeIs,safeEs,hazIs,hazEs,...}) =
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 (writeln"Introduction rules";  prths hazIs;
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  writeln"Safe introduction rules";  prths safeIs;
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  writeln"Elimination rules";  prths hazEs;
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  writeln"Safe elimination rules";  prths safeEs;
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  ());
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fun (CS{safeIs,safeEs,hazIs,hazEs,...}) addSIs ths =
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  make_cs {safeIs=ths@safeIs, safeEs=safeEs, hazIs=hazIs, hazEs=hazEs};
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fun (CS{safeIs,safeEs,hazIs,hazEs,...}) addSEs ths =
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  make_cs {safeIs=safeIs, safeEs=ths@safeEs, hazIs=hazIs, hazEs=hazEs};
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fun cs addSDs ths = cs addSEs (map make_elim ths);
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fun (CS{safeIs,safeEs,hazIs,hazEs,...}) addIs ths =
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  make_cs {safeIs=safeIs, safeEs=safeEs, hazIs=ths@hazIs, hazEs=hazEs};
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fun (CS{safeIs,safeEs,hazIs,hazEs,...}) addEs ths =
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  make_cs {safeIs=safeIs, safeEs=safeEs, hazIs=hazIs, hazEs=ths@hazEs};
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fun cs addDs ths = cs addEs (map make_elim ths);
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(*** Simple tactics for theorem proving ***)
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(*Attack subgoals using safe inferences*)
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fun safe_step_tac (CS{safe0_brls,safep_brls,...}) = 
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  FIRST' [uniq_assume_tac,
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          uniq_mp_tac,
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          biresolve_tac safe0_brls,
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          FIRST' hyp_subst_tacs,
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          biresolve_tac safep_brls] ;
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(*Repeatedly attack subgoals using safe inferences*)
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fun safe_tac cs = DETERM (REPEAT_FIRST (safe_step_tac cs));
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(*These steps could instantiate variables and are therefore unsafe.*)
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val inst_step_tac = assume_tac APPEND' contr_tac;
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(*Single step for the prover.  FAILS unless it makes progress. *)
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fun step_tac (cs as (CS{haz_brls,...})) i = 
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  FIRST [safe_tac cs,
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         inst_step_tac i,
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         biresolve_tac haz_brls i];
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(*** The following tactics all fail unless they solve one goal ***)
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(*Dumb but fast*)
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fun fast_tac cs = SELECT_GOAL (DEPTH_SOLVE (step_tac cs 1));
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(*Slower but smarter than fast_tac*)
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fun best_tac cs = 
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  SELECT_GOAL (BEST_FIRST (has_fewer_prems 1, sizef) (step_tac cs 1));
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(*Using a "safe" rule to instantiate variables is unsafe.  This tactic
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  allows backtracking from "safe" rules to "unsafe" rules here.*)
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fun slow_step_tac (cs as (CS{haz_brls,...})) i = 
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    safe_tac cs ORELSE (assume_tac i APPEND biresolve_tac haz_brls i);
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end; 
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end;