(*  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. The supplied proof state st,

  e.g. from conj_forward, should have the form

    "[| P' ==> ?P; Q' ==> ?Q |] ==> ?P & ?Q"

  and the effect should be to instantiate ?P and ?Q with normalized versions of P' and Q'.*)

fun forward_res nf st =

  let fun forward_tacf [prem] = rtac (nf prem) 1

        | forward_tacf prems = 

            error ("Bad proof state in forward_res, please inform lcp@cl.cam.ac.uk:\n" ^

                   string_of_thm st ^

                   "\nPremises:\n" ^

                   cat_lines (map string_of_thm prems))

  in

    case Seq.pull (ALLGOALS (METAHYPS forward_tacf) st)

    of SOME(th,_) => th

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

  end;

(*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 ***)

val max_clauses = ref 20;

fun sum x y = if x < !max_clauses andalso y < !max_clauses then x+y else !max_clauses;

fun prod x y = if x < !max_clauses andalso y < !max_clauses then x*y else !max_clauses;

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

fun signed_nclauses b (Const("Trueprop",_) $ t) = signed_nclauses b t

  | signed_nclauses b (Const("Not",_) $ t) = signed_nclauses (not b) t

  | signed_nclauses b (Const("op &",_) $ t $ u) = 

      if b then sum (signed_nclauses b t) (signed_nclauses b u)

           else prod (signed_nclauses b t) (signed_nclauses b u)

  | signed_nclauses b (Const("op |",_) $ t $ u) = 

      if b then prod (signed_nclauses b t) (signed_nclauses b u)

           else sum (signed_nclauses b t) (signed_nclauses b u)

  | signed_nclauses b (Const("op -->",_) $ t $ u) = 

      if b then prod (signed_nclauses (not b) t) (signed_nclauses b u)

           else sum (signed_nclauses (not b) t) (signed_nclauses b u)

  | signed_nclauses b (Const("op =", Type ("fun", [T, _])) $ t $ u) = 

      if T = HOLogic.boolT then (*Boolean equality is if-and-only-if*)

	  if b then sum (prod (signed_nclauses (not b) t) (signed_nclauses b u))

			(prod (signed_nclauses (not b) u) (signed_nclauses b t))

	       else sum (prod (signed_nclauses b t) (signed_nclauses b u))

			(prod (signed_nclauses (not b) t) (signed_nclauses (not b) u))

      else 1 

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

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

  | signed_nclauses _ _ = 1; (* literal *)

val nclauses = signed_nclauses true;

fun too_many_clauses t = nclauses t >= !max_clauses;

(*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","Code_Generator.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 (can HOLogic.dest_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 too_many_clauses (concl_of th) 

    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 ****)

fun is_left (Const ("Trueprop", _) $ 

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

  | is_left _ = false;

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

fun assoc_right th = 

  if is_left (prop_of th) then assoc_right (th RS disj_assoc)

  else 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. 

  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);

fun rigid t = not (is_Var (head_of t));

fun ok4horn (Const ("Trueprop",_) $ (Const ("op |", _) $ t $ _)) = rigid t

  | ok4horn (Const ("Trueprop",_) $ t) = rigid t

  | ok4horn _ = false;

(*Create a meta-level Horn clause*)

fun make_horn crules th = 

  if ok4horn (concl_of th) 

  then make_horn crules (tryres(th,crules)) handle THM _ => th

  else 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, PureThy.put_name_hint (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 ok4nnf (Const ("Trueprop",_) $ (Const ("Not", _) $ t)) = rigid t

  | ok4nnf (Const ("Trueprop",_) $ t) = rigid t

  | ok4nnf _ = false;

fun make_nnf1 th = 

  if ok4nnf (concl_of th) 

  then 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

  else 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 = case prems_of th of

    [] => th |> rewrite_rule (map safe_mk_meta_eq nnf_simps)

	     |> simplify nnf_ss  

	     |> make_nnf1

  | _ => raise THM ("make_nnf: premises in argument", 0, [th]);

(*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.expand 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 => 

	     let val horns = make_horns (cls@ths)

	         val _ = if !Output.show_debug_msgs 

	                 then Output.debug ("meson method called:\n" ^ 

	     	                  space_implode "\n" (map string_of_thm (cls@ths)) ^ 

	     	                  "\nclauses:\n" ^ 

	     	                  space_implode "\n" (map string_of_thm horns))

	     	         else ()

	     in THEN_ITER_DEEPEN (resolve_tac goes 1) (has_fewer_prems 1) (prolog_step_tac' horns)

	     end

 );

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 = 

  negated_asm_of_head (make_horn resolution_clause_rules 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_setup =

  Method.add_methods

    [("skolemize", Method.no_args (Method.SIMPLE_METHOD' skolemize_tac),

      "Skolemization into existential quantifiers"),

     ("make_clauses", Method.no_args (Method.SIMPLE_METHOD' make_clauses_tac), 

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

end;

structure BasicMeson: BASIC_MESON = Meson;

open BasicMeson;