(*  Title:      HOL/Tools/meson.ML

     ID:         $Id$

     Author:     Lawrence C Paulson, Cambridge University Computer Laboratory

     Copyright   1992  University of Cambridge

 The MESON resolution proof procedure for HOL.

 When making clauses, avoids using the rewriter -- instead uses RS recursively

 NEED TO SORT LITERALS BY # OF VARS, USING ==>I/E.  ELIMINATES NEED FOR

 FUNCTION nodups -- if done to goal clauses too!

 *)

 signature BASIC_MESON =

 sig

   val size_of_subgoals	: thm -> int

   val make_cnf		: thm list -> thm -> thm list

   val finish_cnf	: thm list -> thm list

   val make_nnf		: thm -> thm

   val make_nnf1		: thm -> thm

   val skolemize		: thm -> thm

   val make_clauses	: thm list -> thm list

   val make_horns	: thm list -> thm list

   val best_prolog_tac	: (thm -> int) -> thm list -> tactic

   val depth_prolog_tac	: thm list -> tactic

   val gocls		: thm list -> thm list

   val skolemize_prems_tac	: thm list -> int -> tactic

   val MESON		: (thm list -> tactic) -> int -> tactic

   val best_meson_tac	: (thm -> int) -> int -> tactic

   val safe_best_meson_tac	: int -> tactic

   val depth_meson_tac	: int -> tactic

   val prolog_step_tac'	: thm list -> int -> tactic

   val iter_deepen_prolog_tac	: thm list -> tactic

   val iter_deepen_meson_tac	: thm list -> int -> tactic

   val meson_tac		: int -> tactic

   val negate_head	: thm -> thm

   val select_literal	: int -> thm -> thm

   val skolemize_tac	: int -> tactic

   val make_clauses_tac	: int -> tactic

 end

 structure Meson =

 struct

 val not_conjD = thm "meson_not_conjD";

 val not_disjD = thm "meson_not_disjD";

 val not_notD = thm "meson_not_notD";

 val not_allD = thm "meson_not_allD";

 val not_exD = thm "meson_not_exD";

 val imp_to_disjD = thm "meson_imp_to_disjD";

 val not_impD = thm "meson_not_impD";

 val iff_to_disjD = thm "meson_iff_to_disjD";

 val not_iffD = thm "meson_not_iffD";

 val conj_exD1 = thm "meson_conj_exD1";

 val conj_exD2 = thm "meson_conj_exD2";

 val disj_exD = thm "meson_disj_exD";

 val disj_exD1 = thm "meson_disj_exD1";

 val disj_exD2 = thm "meson_disj_exD2";

 val disj_assoc = thm "meson_disj_assoc";

 val disj_comm = thm "meson_disj_comm";

 val disj_FalseD1 = thm "meson_disj_FalseD1";

 val disj_FalseD2 = thm "meson_disj_FalseD2";

 val depth_limit = ref 2000;

 (**** Operators for forward proof ****)

 (** First-order Resolution **)

 fun typ_pair_of (ix, (sort,ty)) = (TVar (ix,sort), ty);

 fun term_pair_of (ix, (ty,t)) = (Var (ix,ty), t);

 val Envir.Envir {asol = tenv0, iTs = tyenv0, ...} = Envir.empty 0

 (*FIXME: currently does not "rename variables apart"*)

 fun first_order_resolve thA thB =

   let val thy = theory_of_thm thA

       val tmA = concl_of thA

       fun match pat = Pattern.first_order_match thy (pat,tmA) (tyenv0,tenv0)

       val Const("==>",_) $ tmB $ _ = prop_of thB

       val (tyenv,tenv) = match tmB

       val ct_pairs = map (pairself (cterm_of thy) o term_pair_of) (Vartab.dest tenv)

   in  thA RS (cterm_instantiate ct_pairs thB)  end

   handle _ => raise THM ("first_order_resolve", 0, [thA,thB]);

 (*raises exception if no rules apply -- unlike RL*)

 fun tryres (th, rls) = 

   let fun tryall [] = raise THM("tryres", 0, th::rls)

         | tryall (rl::rls) = (th RS rl handle THM _ => tryall rls)

   in  tryall rls  end;

 (*Permits forward proof from rules that discharge assumptions*)

 fun forward_res nf st =

   case Seq.pull (ALLGOALS (METAHYPS (fn [prem] => rtac (nf prem) 1)) st)

   of SOME(th,_) => th

    | NONE => raise THM("forward_res", 0, [st]);

 (*Are any of the logical connectives in "bs" present in the term?*)

 fun has_conns bs =

   let fun has (Const(a,_)) = false

         | has (Const("Trueprop",_) $ p) = has p

         | has (Const("Not",_) $ p) = has p

         | has (Const("op |",_) $ p $ q) = member (op =) bs "op |" orelse has p orelse has q

         | has (Const("op &",_) $ p $ q) = member (op =) bs "op &" orelse has p orelse has q

         | has (Const("All",_) $ Abs(_,_,p)) = member (op =) bs "All" orelse has p

         | has (Const("Ex",_) $ Abs(_,_,p)) = member (op =) bs "Ex" orelse has p

 	| has _ = false

   in  has  end;

 (**** Clause handling ****)

 fun literals (Const("Trueprop",_) $ P) = literals P

   | literals (Const("op |",_) $ P $ Q) = literals P @ literals Q

   | literals (Const("Not",_) $ P) = [(false,P)]

   | literals P = [(true,P)];

 (*number of literals in a term*)

 val nliterals = length o literals;

 (*** Tautology Checking ***)

 fun signed_lits_aux (Const ("op |", _) $ P $ Q) (poslits, neglits) = 

       signed_lits_aux Q (signed_lits_aux P (poslits, neglits))

   | signed_lits_aux (Const("Not",_) $ P) (poslits, neglits) = (poslits, P::neglits)

   | signed_lits_aux P (poslits, neglits) = (P::poslits, neglits);

 fun signed_lits th = signed_lits_aux (HOLogic.dest_Trueprop (concl_of th)) ([],[]);

 (*Literals like X=X are tautologous*)

 fun taut_poslit (Const("op =",_) $ t $ u) = t aconv u

   | taut_poslit (Const("True",_)) = true

   | taut_poslit _ = false;

 fun is_taut th =

   let val (poslits,neglits) = signed_lits th

   in  exists taut_poslit poslits

       orelse

       exists (member (op aconv) neglits) (HOLogic.false_const :: poslits)

   end

   handle TERM _ => false;	(*probably dest_Trueprop on a weird theorem*)		      

 (*** To remove trivial negated equality literals from clauses ***)

 (*They are typically functional reflexivity axioms and are the converses of

   injectivity equivalences*)

 val not_refl_disj_D = thm"meson_not_refl_disj_D";

 (*Is either term a Var that does not properly occur in the other term?*)

 fun eliminable (t as Var _, u) = t aconv u orelse not (Logic.occs(t,u))

   | eliminable (u, t as Var _) = t aconv u orelse not (Logic.occs(t,u))

   | eliminable _ = false;

 fun refl_clause_aux 0 th = th

   | refl_clause_aux n th =

        case HOLogic.dest_Trueprop (concl_of th) of

 	  (Const ("op |", _) $ (Const ("op |", _) $ _ $ _) $ _) => 

             refl_clause_aux n (th RS disj_assoc)    (*isolate an atom as first disjunct*)

 	| (Const ("op |", _) $ (Const("Not",_) $ (Const("op =",_) $ t $ u)) $ _) => 

 	    if eliminable(t,u) 

 	    then refl_clause_aux (n-1) (th RS not_refl_disj_D)  (*Var inequation: delete*)

 	    else refl_clause_aux (n-1) (th RS disj_comm)  (*not between Vars: ignore*)

 	| (Const ("op |", _) $ _ $ _) => refl_clause_aux n (th RS disj_comm)

 	| _ => (*not a disjunction*) th;

 fun notequal_lits_count (Const ("op |", _) $ P $ Q) = 

       notequal_lits_count P + notequal_lits_count Q

   | notequal_lits_count (Const("Not",_) $ (Const("op =",_) $ _ $ _)) = 1

   | notequal_lits_count _ = 0;

 (*Simplify a clause by applying reflexivity to its negated equality literals*)

 fun refl_clause th = 

   let val neqs = notequal_lits_count (HOLogic.dest_Trueprop (concl_of th))

   in  zero_var_indexes (refl_clause_aux neqs th)  end

   handle TERM _ => th;	(*probably dest_Trueprop on a weird theorem*)		      

 (*** The basic CNF transformation ***)

 (*Estimate the number of clauses in order to detect infeasible theorems*)

 fun nclauses (Const("Trueprop",_) $ t) = nclauses t

   | nclauses (Const("op &",_) $ t $ u) = nclauses t + nclauses u

   | nclauses (Const("Ex", _) $ Abs (_,_,t)) = nclauses t

   | nclauses (Const("All",_) $ Abs (_,_,t)) = nclauses t

   | nclauses (Const("op |",_) $ t $ u) = nclauses t * nclauses u

   | nclauses _ = 1; (* literal *)

 (*Replaces universally quantified variables by FREE variables -- because

   assumptions may not contain scheme variables.  Later, call "generalize". *)

 fun freeze_spec th =

   let val newname = gensym "mes_"

       val spec' = read_instantiate [("x", newname)] spec

   in  th RS spec'  end;

 (*Used with METAHYPS below. There is one assumption, which gets bound to prem

   and then normalized via function nf. The normal form is given to resolve_tac,

   presumably to instantiate a Boolean variable.*)

 fun resop nf [prem] = resolve_tac (nf prem) 1;

 (*Any need to extend this list with 

   "HOL.type_class","OperationalEquality.eq_class","ProtoPure.term"?*)

 val has_meta_conn = 

     exists_Const (fn (c,_) => c mem_string ["==", "==>", "all", "prop"]);

 fun apply_skolem_ths (th, rls) = 

   let fun tryall [] = raise THM("apply_skolem_ths", 0, th::rls)

         | tryall (rl::rls) = (first_order_resolve th rl handle THM _ => tryall rls)

   in  tryall rls  end;

 (*Conjunctive normal form, adding clauses from th in front of ths (for foldr).

   Strips universal quantifiers and breaks up conjunctions.

   Eliminates existential quantifiers using skoths: Skolemization theorems.*)

 fun cnf skoths (th,ths) =

   let fun cnf_aux (th,ths) =

   	if not (HOLogic.is_Trueprop (prop_of th)) then ths (*meta-level: ignore*)

         else if not (has_conns ["All","Ex","op &"] (prop_of th))  

 	then th::ths (*no work to do, terminate*)

 	else case head_of (HOLogic.dest_Trueprop (concl_of th)) of

 	    Const ("op &", _) => (*conjunction*)

 		cnf_aux (th RS conjunct1, cnf_aux (th RS conjunct2, ths))

 	  | Const ("All", _) => (*universal quantifier*)

 	        cnf_aux (freeze_spec th,  ths)

 	  | Const ("Ex", _) => 

 	      (*existential quantifier: Insert Skolem functions*)

 	      cnf_aux (apply_skolem_ths (th,skoths), ths)

 	  | Const ("op |", _) => (*disjunction*)

 	      let val tac =

 		  (METAHYPS (resop cnf_nil) 1) THEN

 		   (fn st' => st' |> METAHYPS (resop cnf_nil) 1)

 	      in  Seq.list_of (tac (th RS disj_forward)) @ ths  end 

 	  | _ => (*no work to do*) th::ths 

       and cnf_nil th = cnf_aux (th,[])

   in 

     if nclauses (concl_of th) > 20 

     then (Output.debug ("cnf is ignoring: " ^ string_of_thm th); ths)

     else cnf_aux (th,ths)

   end;

 (*Convert all suitable free variables to schematic variables, 

   but don't discharge assumptions.*)

 fun generalize th = Thm.varifyT (forall_elim_vars 0 (forall_intr_frees th));

 fun make_cnf skoths th = cnf skoths (th, []);

 (*Generalization, removal of redundant equalities, removal of tautologies.*)

 fun finish_cnf ths = filter (not o is_taut) (map (refl_clause o generalize) ths);

 (**** Removal of duplicate literals ****)

 (*Forward proof, passing extra assumptions as theorems to the tactic*)

 fun forward_res2 nf hyps st =

   case Seq.pull

 	(REPEAT

 	 (METAHYPS (fn major::minors => rtac (nf (minors@hyps) major) 1) 1)

 	 st)

   of SOME(th,_) => th

    | NONE => raise THM("forward_res2", 0, [st]);

 (*Remove duplicates in P|Q by assuming ~P in Q

   rls (initially []) accumulates assumptions of the form P==>False*)

 fun nodups_aux rls th = nodups_aux rls (th RS disj_assoc)

     handle THM _ => tryres(th,rls)

     handle THM _ => tryres(forward_res2 nodups_aux rls (th RS disj_forward2),

 			   [disj_FalseD1, disj_FalseD2, asm_rl])

     handle THM _ => th;

 (*Remove duplicate literals, if there are any*)

 fun nodups th =

   if has_duplicates (op =) (literals (prop_of th))

     then nodups_aux [] th

     else th;

 (**** Generation of contrapositives ****)

 (*Associate disjuctions to right -- make leftmost disjunct a LITERAL*)

 fun assoc_right th = assoc_right (th RS disj_assoc)

 	handle THM _ => th;

 (*Must check for negative literal first!*)

 val clause_rules = [disj_assoc, make_neg_rule, make_pos_rule];

 (*For ordinary resolution. *)

 val resolution_clause_rules = [disj_assoc, make_neg_rule', make_pos_rule'];

 (*Create a goal or support clause, conclusing False*)

 fun make_goal th =   (*Must check for negative literal first!*)

     make_goal (tryres(th, clause_rules))

   handle THM _ => tryres(th, [make_neg_goal, make_pos_goal]);

 (*Sort clauses by number of literals*)

 fun fewerlits(th1,th2) = nliterals(prop_of th1) < nliterals(prop_of th2);

 fun sort_clauses ths = sort (make_ord fewerlits) ths;

 (*True if the given type contains bool anywhere*)

 fun has_bool (Type("bool",_)) = true

   | has_bool (Type(_, Ts)) = exists has_bool Ts

   | has_bool _ = false;

 (*Is the string the name of a connective? Really only | and Not can remain, 

   since this code expects to be called on a clause form.*)  

 val is_conn = member (op =)

     ["Trueprop", "op &", "op |", "op -->", "Not", 

      "All", "Ex", "Ball", "Bex"];

 (*True if the term contains a function--not a logical connective--where the type 

   of any argument contains bool.*)

 val has_bool_arg_const = 

     exists_Const

       (fn (c,T) => not(is_conn c) andalso exists (has_bool) (binder_types T));

 (*Raises an exception if any Vars in the theorem mention type bool; they

   could cause make_horn to loop! Also rejects functions whose arguments are 

   Booleans or other functions.*)

 fun is_fol_term t =

     not (exists (has_bool o fastype_of) (term_vars t)  orelse

 	 not (Term.is_first_order ["all","All","Ex"] t) orelse

 	 has_bool_arg_const t  orelse  

 	 has_meta_conn t);

 (*Create a meta-level Horn clause*)

 fun make_horn crules th = make_horn crules (tryres(th,crules))

 			  handle THM _ => th;

 (*Generate Horn clauses for all contrapositives of a clause. The input, th,

   is a HOL disjunction.*)

 fun add_contras crules (th,hcs) =

   let fun rots (0,th) = hcs

 	| rots (k,th) = zero_var_indexes (make_horn crules th) ::

 			rots(k-1, assoc_right (th RS disj_comm))

   in case nliterals(prop_of th) of

 	1 => th::hcs

       | n => rots(n, assoc_right th)

   end;

 (*Use "theorem naming" to label the clauses*)

 fun name_thms label =

     let fun name1 (th, (k,ths)) =

 	  (k-1, Thm.name_thm (label ^ string_of_int k, th) :: ths)

     in  fn ths => #2 (foldr name1 (length ths, []) ths)  end;

 (*Is the given disjunction an all-negative support clause?*)

 fun is_negative th = forall (not o #1) (literals (prop_of th));

 val neg_clauses = List.filter is_negative;

 (***** MESON PROOF PROCEDURE *****)

 fun rhyps (Const("==>",_) $ (Const("Trueprop",_) $ A) $ phi,

 	   As) = rhyps(phi, A::As)

   | rhyps (_, As) = As;

 (** Detecting repeated assumptions in a subgoal **)

 (*The stringtree detects repeated assumptions.*)

 fun ins_term (net,t) = Net.insert_term (op aconv) (t,t) net;

 (*detects repetitions in a list of terms*)

 fun has_reps [] = false

   | has_reps [_] = false

   | has_reps [t,u] = (t aconv u)

   | has_reps ts = (Library.foldl ins_term (Net.empty, ts);  false)

 		  handle Net.INSERT => true;

 (*Like TRYALL eq_assume_tac, but avoids expensive THEN calls*)

 fun TRYING_eq_assume_tac 0 st = Seq.single st

   | TRYING_eq_assume_tac i st =

        TRYING_eq_assume_tac (i-1) (eq_assumption i st)

        handle THM _ => TRYING_eq_assume_tac (i-1) st;

 fun TRYALL_eq_assume_tac st = TRYING_eq_assume_tac (nprems_of st) st;

 (*Loop checking: FAIL if trying to prove the same thing twice

   -- if *ANY* subgoal has repeated literals*)

 fun check_tac st =

   if exists (fn prem => has_reps (rhyps(prem,[]))) (prems_of st)

   then  Seq.empty  else  Seq.single st;

 (* net_resolve_tac actually made it slower... *)

 fun prolog_step_tac horns i =

     (assume_tac i APPEND resolve_tac horns i) THEN check_tac THEN

     TRYALL_eq_assume_tac;

 (*Sums the sizes of the subgoals, ignoring hypotheses (ancestors)*)

 fun addconcl(prem,sz) = size_of_term(Logic.strip_assums_concl prem) + sz

 fun size_of_subgoals st = foldr addconcl 0 (prems_of st);

 (*Negation Normal Form*)

 val nnf_rls = [imp_to_disjD, iff_to_disjD, not_conjD, not_disjD,

                not_impD, not_iffD, not_allD, not_exD, not_notD];

 fun make_nnf1 th = make_nnf1 (tryres(th, nnf_rls))

     handle THM _ =>

         forward_res make_nnf1

            (tryres(th, [conj_forward,disj_forward,all_forward,ex_forward]))

     handle THM _ => th;

 (*The simplification removes defined quantifiers and occurrences of True and False. 

   nnf_ss also includes the one-point simprocs,

   which are needed to avoid the various one-point theorems from generating junk clauses.*)

 val nnf_simps =

      [simp_implies_def, Ex1_def, Ball_def, Bex_def, if_True, 

       if_False, if_cancel, if_eq_cancel, cases_simp];

 val nnf_extra_simps =

       thms"split_ifs" @ ex_simps @ all_simps @ simp_thms;

 val nnf_ss =

     HOL_basic_ss addsimps nnf_extra_simps 

                  addsimprocs [defALL_regroup,defEX_regroup,neq_simproc,let_simproc];

 fun make_nnf th = th |> rewrite_rule (map safe_mk_meta_eq nnf_simps)

                      |> simplify nnf_ss  (*But this doesn't simplify premises...*)

                      |> make_nnf1

 (*Pull existential quantifiers to front. This accomplishes Skolemization for

   clauses that arise from a subgoal.*)

 fun skolemize th =

   if not (has_conns ["Ex"] (prop_of th)) then th

   else (skolemize (tryres(th, [choice, conj_exD1, conj_exD2,

                               disj_exD, disj_exD1, disj_exD2])))

     handle THM _ =>

         skolemize (forward_res skolemize

                    (tryres (th, [conj_forward, disj_forward, all_forward])))

     handle THM _ => forward_res skolemize (th RS ex_forward);

 (*Make clauses from a list of theorems, previously Skolemized and put into nnf.

   The resulting clauses are HOL disjunctions.*)

 fun make_clauses ths =

     (sort_clauses (map (generalize o nodups) (foldr (cnf[]) [] ths)));

 (*Convert a list of clauses (disjunctions) to Horn clauses (contrapositives)*)

 fun make_horns ths =

     name_thms "Horn#"

       (distinct Drule.eq_thm_prop (foldr (add_contras clause_rules) [] ths));

 (*Could simply use nprems_of, which would count remaining subgoals -- no

   discrimination as to their size!  With BEST_FIRST, fails for problem 41.*)

 fun best_prolog_tac sizef horns =

     BEST_FIRST (has_fewer_prems 1, sizef) (prolog_step_tac horns 1);

 fun depth_prolog_tac horns =

     DEPTH_FIRST (has_fewer_prems 1) (prolog_step_tac horns 1);

 (*Return all negative clauses, as possible goal clauses*)

 fun gocls cls = name_thms "Goal#" (map make_goal (neg_clauses cls));

 fun skolemize_prems_tac prems =

     cut_facts_tac (map (skolemize o make_nnf) prems)  THEN'

     REPEAT o (etac exE);

 (*Expand all definitions (presumably of Skolem functions) in a proof state.*)

 fun expand_defs_tac st =

   let val defs = filter (can dest_equals) (#hyps (crep_thm st))

   in  PRIMITIVE (LocalDefs.def_export false defs) st  end;

 (*Basis of all meson-tactics.  Supplies cltac with clauses: HOL disjunctions*)

 fun MESON cltac i st = 

   SELECT_GOAL

     (EVERY [rtac ccontr 1,

 	    METAHYPS (fn negs =>

 		      EVERY1 [skolemize_prems_tac negs,

 			      METAHYPS (cltac o make_clauses)]) 1,

             expand_defs_tac]) i st

   handle THM _ => no_tac st;	(*probably from make_meta_clause, not first-order*)		      

 (** Best-first search versions **)

 (*ths is a list of additional clauses (HOL disjunctions) to use.*)

 fun best_meson_tac sizef =

   MESON (fn cls =>

          THEN_BEST_FIRST (resolve_tac (gocls cls) 1)

                          (has_fewer_prems 1, sizef)

                          (prolog_step_tac (make_horns cls) 1));

 (*First, breaks the goal into independent units*)

 val safe_best_meson_tac =

      SELECT_GOAL (TRY Safe_tac THEN

                   TRYALL (best_meson_tac size_of_subgoals));

 (** Depth-first search version **)

 val depth_meson_tac =

      MESON (fn cls => EVERY [resolve_tac (gocls cls) 1,

                              depth_prolog_tac (make_horns cls)]);

 (** Iterative deepening version **)

 (*This version does only one inference per call;

   having only one eq_assume_tac speeds it up!*)

 fun prolog_step_tac' horns =

     let val (horn0s, hornps) = (*0 subgoals vs 1 or more*)

             take_prefix Thm.no_prems horns

         val nrtac = net_resolve_tac horns

     in  fn i => eq_assume_tac i ORELSE

                 match_tac horn0s i ORELSE  (*no backtracking if unit MATCHES*)

                 ((assume_tac i APPEND nrtac i) THEN check_tac)

     end;

 fun iter_deepen_prolog_tac horns =

     ITER_DEEPEN (has_fewer_prems 1) (prolog_step_tac' horns);

 fun iter_deepen_meson_tac ths =

   MESON (fn cls =>

            case (gocls (cls@ths)) of

            	[] => no_tac  (*no goal clauses*)

               | goes => 

 		 (THEN_ITER_DEEPEN (resolve_tac goes 1)

 				   (has_fewer_prems 1)

 				   (prolog_step_tac' (make_horns (cls@ths)))));

 fun meson_claset_tac ths cs =

   SELECT_GOAL (TRY (safe_tac cs) THEN TRYALL (iter_deepen_meson_tac ths));

 val meson_tac = CLASET' (meson_claset_tac []);

 (**** Code to support ordinary resolution, rather than Model Elimination ****)

 (*Convert a list of clauses (disjunctions) to meta-level clauses (==>), 

   with no contrapositives, for ordinary resolution.*)

 (*Rules to convert the head literal into a negated assumption. If the head

   literal is already negated, then using notEfalse instead of notEfalse'

   prevents a double negation.*)

 val notEfalse = read_instantiate [("R","False")] notE;

 val notEfalse' = rotate_prems 1 notEfalse;

 fun negated_asm_of_head th = 

     th RS notEfalse handle THM _ => th RS notEfalse';

 (*Converting one clause*)

 fun make_meta_clause th = 

   if is_fol_term (prop_of th) 

   then negated_asm_of_head (make_horn resolution_clause_rules th)

   else raise THM("make_meta_clause", 0, [th]);

 fun make_meta_clauses ths =

     name_thms "MClause#"

       (distinct Drule.eq_thm_prop (map make_meta_clause ths));

 (*Permute a rule's premises to move the i-th premise to the last position.*)

 fun make_last i th =

   let val n = nprems_of th 

   in  if 1 <= i andalso i <= n 

       then Thm.permute_prems (i-1) 1 th

       else raise THM("select_literal", i, [th])

   end;

 (*Maps a rule that ends "... ==> P ==> False" to "... ==> ~P" while suppressing

   double-negations.*)

 val negate_head = rewrite_rule [atomize_not, not_not RS eq_reflection];

 (*Maps the clause  [P1,...Pn]==>False to [P1,...,P(i-1),P(i+1),...Pn] ==> ~P*)

 fun select_literal i cl = negate_head (make_last i cl);

 (*Top-level Skolemization. Allows part of the conversion to clauses to be

   expressed as a tactic (or Isar method).  Each assumption of the selected 

   goal is converted to NNF and then its existential quantifiers are pulled

   to the front. Finally, all existential quantifiers are eliminated, 

   leaving !!-quantified variables. Perhaps Safe_tac should follow, but it

   might generate many subgoals.*)

 fun skolemize_tac i st = 

   let val ts = Logic.strip_assums_hyp (List.nth (prems_of st, i-1))

   in 

      EVERY' [METAHYPS

 	    (fn hyps => (cut_facts_tac (map (skolemize o make_nnf) hyps) 1

                          THEN REPEAT (etac exE 1))),

             REPEAT_DETERM_N (length ts) o (etac thin_rl)] i st

   end

   handle Subscript => Seq.empty;

 (*Top-level conversion to meta-level clauses. Each clause has  

   leading !!-bound universal variables, to express generality. To get 

   disjunctions instead of meta-clauses, remove "make_meta_clauses" below.*)

 val make_clauses_tac = 

   SUBGOAL

     (fn (prop,_) =>

      let val ts = Logic.strip_assums_hyp prop

      in EVERY1 

 	 [METAHYPS

 	    (fn hyps => 

               (Method.insert_tac

                 (map forall_intr_vars 

                   (make_meta_clauses (make_clauses hyps))) 1)),

 	  REPEAT_DETERM_N (length ts) o (etac thin_rl)]

      end);

 (*** setup the special skoklemization methods ***)

 (*No CHANGED_PROP here, since these always appear in the preamble*)

 val skolemize_meth = Method.SIMPLE_METHOD' HEADGOAL skolemize_tac;

 val make_clauses_meth = Method.SIMPLE_METHOD' HEADGOAL make_clauses_tac;

 val skolemize_setup =

   Method.add_methods

     [("skolemize", Method.no_args skolemize_meth, 

       "Skolemization into existential quantifiers"),

      ("make_clauses", Method.no_args make_clauses_meth, 

       "Conversion to !!-quantified meta-level clauses")];

 end;

 structure BasicMeson: BASIC_MESON = Meson;

 open BasicMeson;