(*  Title:      FOLP/intprover.ML

    ID:         $Id$

    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory

    Copyright   1992  University of Cambridge

A naive prover for intuitionistic logic

BEWARE OF NAME CLASHES WITH CLASSICAL TACTICS -- use IntPr.fast_tac ...

Completeness (for propositional logic) is proved in 

Roy Dyckhoff.

Contraction-Free Sequent Calculi for IntPruitionistic Logic.

J. Symbolic Logic (in press)

*)

signature INT_PROVER = 

  sig

  val best_tac: int -> tactic

  val fast_tac: int -> tactic

  val inst_step_tac: int -> tactic

  val safe_step_tac: int -> tactic

  val safe_brls: (bool * thm) list

  val safe_tac: tactic

  val step_tac: int -> tactic

  val haz_brls: (bool * thm) list

  end;

structure IntPr : INT_PROVER   = 

struct

(*Negation is treated as a primitive symbol, with rules notI (introduction),

  not_to_imp (converts the assumption ~P to P-->False), and not_impE

  (handles double negations).  Could instead rewrite by not_def as the first

  step of an intuitionistic proof.

*)

val safe_brls = sort (make_ord lessb)

    [ (true,FalseE), (false,TrueI), (false,refl),

      (false,impI), (false,notI), (false,allI),

      (true,conjE), (true,exE),

      (false,conjI), (true,conj_impE),

      (true,disj_impE), (true,disjE), 

      (false,iffI), (true,iffE), (true,not_to_imp) ];

val haz_brls =

    [ (false,disjI1), (false,disjI2), (false,exI), 

      (true,allE), (true,not_impE), (true,imp_impE), (true,iff_impE),

      (true,all_impE), (true,ex_impE), (true,impE) ];

(*0 subgoals vs 1 or more: the p in safep is for positive*)

val (safe0_brls, safep_brls) =

    List.partition (curry (op =) 0 o subgoals_of_brl) safe_brls;

(*Attack subgoals using safe inferences*)

val safe_step_tac = FIRST' [uniq_assume_tac,

                            int_uniq_mp_tac,

                            biresolve_tac safe0_brls,

                            hyp_subst_tac,

                            biresolve_tac safep_brls] ;

(*Repeatedly attack subgoals using safe inferences*)

val safe_tac = DETERM (REPEAT_FIRST safe_step_tac);

(*These steps could instantiate variables and are therefore unsafe.*)

val inst_step_tac = assume_tac APPEND' mp_tac;

(*One safe or unsafe step. *)

fun step_tac i = FIRST [safe_tac, inst_step_tac i, biresolve_tac haz_brls i];

(*Dumb but fast*)

val fast_tac = SELECT_GOAL (DEPTH_SOLVE (step_tac 1));

(*Slower but smarter than fast_tac*)

val best_tac = 

  SELECT_GOAL (BEST_FIRST (has_fewer_prems 1, size_of_thm) (step_tac 1));

end;