src/Provers/blast.ML

Implementation of blast_tac: fast tableau prover

(*  ID:         $Id$
use "/homes/lcp/Isa/new/blast.ML";
  SKOLEMIZES ReplaceI WRONGLY: allow new vars in prems, or forbid such rules??
  Needs explicit instantiation of assumptions?  (#55 takes 32s)

*)


proof_timing:=true;
print_depth 20;

structure List = List_;

(*Should be a type abbreviation?*)
type netpair = (int*(bool*thm)) Net.net * (int*(bool*thm)) Net.net;


(*Assumptions about constants:
  --The negation symbol is "Not"
  --The equality symbol is "op ="
  --The is-true judgement symbol is "Trueprop"
  --There are no constants named "*Goal* or "*False*"
*)
signature BLAST_DATA =
  sig
  type claset
  val notE		: thm		(* [| ~P;  P |] ==> R *)
  val ccontr		: thm		
  val contr_tac 	: int -> tactic
  val dup_intr		: thm -> thm
  val vars_gen_hyp_subst_tac : bool -> int -> tactic
  val claset		: claset ref
  val rep_claset	: 
      claset -> {safeIs: thm list, safeEs: thm list, 
		 hazIs: thm list, hazEs: thm list,
		 uwrapper: (int -> tactic) -> (int -> tactic),
		 swrapper: (int -> tactic) -> (int -> tactic),
		 safe0_netpair: netpair, safep_netpair: netpair,
		 haz_netpair: netpair, dup_netpair: netpair}
  end;


signature BLAST =
  sig
  type claset
  val depth_tac 		: claset -> int -> int -> tactic
  val blast_tac 		: claset -> int -> tactic
  val Blast_tac 		: int -> tactic
  end;


functor BlastFun(Data: BLAST_DATA): BLAST = 
struct

type claset = Data.claset;

val trace = ref false;

datatype term = 
    Const of string
  | OConst of string * int
  | Skolem of string * term option ref list
  | Free  of string
  | Var   of term option ref
  | Bound of int
  | Abs   of string*term
  | op $  of term*term;


exception DEST_EQ;

  (*Take apart an equality (plain or overloaded).  NO constant Trueprop*)
  fun dest_eq (Const  "op ="     $ t $ u) = (t,u)
    | dest_eq (OConst("op =",_)  $ t $ u) = (t,u)
    | dest_eq _                      = raise DEST_EQ;

(** Basic syntactic operations **)

fun is_Var (Var _) = true
  | is_Var _ = false;

fun dest_Var (Var x) =  x;


fun rand (f$x) = x;

(* maps   (f, [t1,...,tn])  to  f(t1,...,tn) *)
val list_comb : term * term list -> term = foldl (op $);


(* maps   f(t1,...,tn)  to  (f, [t1,...,tn]) ; naturally tail-recursive*)
fun strip_comb u : term * term list = 
    let fun stripc (f$t, ts) = stripc (f, t::ts)
        |   stripc  x =  x 
    in  stripc(u,[])  end;


(* maps   f(t1,...,tn)  to  f , which is never a combination *)
fun head_of (f$t) = head_of f
  | head_of u = u;


(** Particular constants **)

fun negate P = Const"Not" $ P;

fun mkGoal P = Const"*Goal*" $ P;

fun isGoal (Const"*Goal*" $ _) = true
  | isGoal _                   = false;

val Trueprop = Term.Const("Trueprop", Type("o",[])-->propT);
fun mk_tprop P = Term.$ (Trueprop, P);

fun isTrueprop (Term.Const("Trueprop",_)) = true
  | isTrueprop _                          = false;


(** Dealing with overloaded constants **)

(*Result is a symbol table, indexed by names of overloaded constants.
  Each constant maps to a list of (pattern,Blast.Const) pairs.
  Any Term.Const that matches a pattern gets replaced by the Blast.Const.
*)
fun addConsts (t as Term.Const(a,_), tab) =
     (case Symtab.lookup (tab,a) of
	  None    => tab  (*ignore: not a constant that we are looking for*)
	| Some patList => 
	      (case gen_assoc (op aconv) (patList, t) of
		  None => Symtab.update
		           ((a, (t, OConst (a, length patList)) :: patList), 
			    tab)
		 | _    => tab))
  | addConsts (Term.Abs(_,_,body), tab) = addConsts (body, tab)
  | addConsts (Term.$ (t,u), tab) = addConsts (t, addConsts (u, tab))
  | addConsts (_,            tab) = tab (*ignore others*);


fun addRules (rls,tab) = foldr addConsts (map (#prop o rep_thm) rls, tab);

fun declConst (a,tab) = Symtab.update((a,[]), tab);

(*maps the name of each overloaded constant to a list of archetypal constants,
  which may be polymorphic.*)
local
val overLoadTab = ref (Symtab.null : (Term.term * term) list Symtab.table)
    (*The alists in this table should only be increased*)
in

fun declConsts (names, rls) =
    overLoadTab := addRules (rls, foldr declConst (names, !overLoadTab));


(*Convert a possibly overloaded Term.Const to a Blast.Const*)
fun fromConst tsig (t as Term.Const (a,_)) =
  let fun find []                  = Const a
	| find ((pat,t')::patList) =
		if Pattern.matches tsig (pat,t) then t' 
		else find patList
  in  case Symtab.lookup(!overLoadTab, a) of
	   None         => Const a
	 | Some patList => find patList
  end;
end;


(*Tests whether 2 terms are alpha-convertible; chases instantiations*)
fun (Const a)      aconv (Const b)      = a=b
  | (OConst ai)    aconv (OConst bj)    = ai=bj
  | (Skolem (a,_)) aconv (Skolem (b,_)) = a=b  (*arglists must then be equal*)
  | (Free a)       aconv (Free b)       = a=b
  | (Var(ref(Some t))) aconv u          = t aconv u
  | t aconv (Var(ref(Some u)))          = t aconv u
  | (Var v)        aconv (Var w)        = v=w	(*both Vars are un-assigned*)
  | (Bound i)      aconv (Bound j)      = i=j
  | (Abs(_,t))     aconv (Abs(_,u))     = t aconv u
  | (f$t)          aconv (g$u)          = (f aconv g) andalso (t aconv u)
  | _ aconv _  =  false;


fun mem_term (_, [])     = false
  | mem_term (t, t'::ts) = t aconv t' orelse mem_term(t,ts);

fun ins_term(t,ts) = if mem_term(t,ts) then ts else t :: ts;

fun mem_var (v: term option ref, []) = false
  | mem_var (v, v'::vs)              = v=v' orelse mem_var(v,vs);

fun ins_var(v,vs) = if mem_var(v,vs) then vs else v :: vs;


(** Vars **)

(*Accumulates the Vars in the term, suppressing duplicates*)
fun add_term_vars (Skolem(a,args),	vars) = add_vars_vars(args,vars)
  | add_term_vars (Var (v as ref None),	vars) = ins_var (v, vars)
  | add_term_vars (Var (ref (Some u)), vars)  = add_term_vars(u,vars)
  | add_term_vars (Abs (_,body),	vars) = add_term_vars(body,vars)
  | add_term_vars (f$t,	vars) =  add_term_vars (f, add_term_vars(t, vars))
  | add_term_vars (_,	vars) = vars
(*Term list version.  [The fold functionals are slow]*)
and add_terms_vars ([],    vars) = vars
  | add_terms_vars (t::ts, vars) = add_terms_vars (ts, add_term_vars(t,vars))
(*Var list version.*)
and add_vars_vars ([],    vars) = vars
  | add_vars_vars (ref (Some u) :: vs, vars) = 
	add_vars_vars (vs, add_term_vars(u,vars))
  | add_vars_vars (v::vs, vars) =   (*v must be a ref None*)
	add_vars_vars (vs, ins_var (v, vars));


(*Chase assignments in "vars"; return a list of unassigned variables*)
fun vars_in_vars vars = add_vars_vars(vars,[]);



(*increment a term's non-local bound variables
     inc is  increment for bound variables
     lev is  level at which a bound variable is considered 'loose'*)
fun incr_bv (inc, lev, u as Bound i) = if i>=lev then Bound(i+inc) else u 
  | incr_bv (inc, lev, Abs(a,body)) = Abs(a, incr_bv(inc,lev+1,body))
  | incr_bv (inc, lev, f$t) = incr_bv(inc,lev,f) $ incr_bv(inc,lev,t)
  | incr_bv (inc, lev, u) = u;

fun incr_boundvars  0  t = t
  | incr_boundvars inc t = incr_bv(inc,0,t);


(*Accumulate all 'loose' bound vars referring to level 'lev' or beyond.
   (Bound 0) is loose at level 0 *)
fun add_loose_bnos (Bound i, lev, js)   = if i<lev then js  
					  else  (i-lev) ins_int js
  | add_loose_bnos (Abs (_,t), lev, js) = add_loose_bnos (t, lev+1, js)
  | add_loose_bnos (f$t, lev, js)       =
	        add_loose_bnos (f, lev, add_loose_bnos (t, lev, js)) 
  | add_loose_bnos (_, _, js)           = js;

fun loose_bnos t = add_loose_bnos (t, 0, []);

fun subst_bound (arg, t) : term = 
  let fun subst (t as Bound i, lev) =
 	    if i<lev then  t    (*var is locally bound*)
	    else  if i=lev then incr_boundvars lev arg
		           else Bound(i-1)  (*loose: change it*)
	| subst (Abs(a,body), lev) = Abs(a, subst(body,lev+1))
	| subst (f$t, lev) =  subst(f,lev)  $  subst(t,lev)
	| subst (t,lev)    = t
  in  subst (t,0)  end;


(*Normalize...but not the bodies of ABSTRACTIONS*)
fun norm t = case t of
    Skolem(a,args)       => Skolem(a, vars_in_vars args)
  | (Var (ref None))     => t
  | (Var (ref (Some u))) => norm u
  | (f $ u) => (case norm f of
		    Abs(_,body) => norm (subst_bound (u, body))
		  | nf => nf $ norm u)
  | _ => t;


(*Weak (one-level) normalize for use in unification*)
fun wkNormAux t = case t of
    (Var v) => (case !v of
		    Some u => wkNorm u
		  | None   => t)
  | (f $ u) => (case wkNormAux f of
		    Abs(_,body) => wkNorm (subst_bound (u, body))
		  | nf          => nf $ u)
  | _ => t
and wkNorm t = case head_of t of
    Const _        => t
  | OConst _       => t
  | Skolem(a,args) => t
  | Free _         => t
  | _              => wkNormAux t;


(*Does variable v occur in u?  For unification.*)
fun varOccur v = 
  let fun occL [] = false	(*same as (exists occ), but faster*)
	| occL (u::us) = occ u orelse occL us
      and occ (Var w) = 
	      v=w orelse
              (case !w of None   => false
	                | Some u => occ u)
        | occ (Skolem(_,args)) = occL (map Var args)
        | occ (Abs(_,u)) = occ u
        | occ (f$u)      = occ u  orelse  occ f
        | occ (_)        = false;
  in  occ  end;

exception UNIFY;

val trail = ref [] : term option ref list ref;
val ntrail = ref 0;


(*Restore the trail to some previous state: for backtracking*)
fun clearTo n =
    while !ntrail>n do
	(hd(!trail) := None;
	 trail := tl (!trail);
	 ntrail := !ntrail - 1);


(*First-order unification with bound variables.  
  "vars" is a list of variables local to the rule and NOT to be put
	on the trail (no point in doing so)
*)
fun unify(vars,t,u) =
    let val n = !ntrail 
	fun update (t as Var v, u) =
	    if t aconv u then ()
	    else if varOccur v u then raise UNIFY 
	    else if mem_var(v, vars) then v := Some u
		 else (*avoid updating Vars in the branch if possible!*)
		      if is_Var u andalso mem_var(dest_Var u, vars)
		      then dest_Var u := Some t
		      else (v := Some u;
			    trail := v :: !trail;  ntrail := !ntrail + 1)
	fun unifyAux (t,u) = 
	    case (wkNorm t,  wkNorm u) of
		(nt as Var v,  nu) => update(nt,nu)
	      | (nu,  nt as Var v) => update(nt,nu)
	      | (Abs(_,t'),  Abs(_,u')) => unifyAux(t',u')
		    (*NB: can yield unifiers having dangling Bound vars!*)
	      | (f$t',  g$u') => (unifyAux(f,g); unifyAux(t',u'))
	      | (nt,  nu)    => if nt aconv nu then () else raise UNIFY
    in  unifyAux(t,u) handle UNIFY => (clearTo n; raise UNIFY)
    end;


(*Convert from "real" terms to prototerms; eta-contract*)
fun fromTerm tsig t =
  let val alist = ref []
      fun from (t as Term.Const _) = fromConst tsig t
	| from (Term.Free  (a,_)) = Free a
	| from (Term.Bound i)     = Bound i
	| from (Term.Var (ixn,T)) = 
	      (case (assoc_string_int(!alist,ixn)) of
		   None => let val t' = Var(ref None)
		           in  alist := (ixn, (t', T)) :: !alist;  t'
			   end
		 | Some (v,_) => v)
	| from (Term.Abs (a,_,u)) = 
	      (case  from u  of
		u' as (f $ Bound 0) => 
		  if (0 mem_int loose_bnos f) then Abs(a,u')
		  else incr_boundvars ~1 f 
	      | u' => Abs(a,u'))
	| from (Term.$ (f,u)) = from f $ from u
  in  from t  end;

(* A1==>...An==>B  goes to  [A1,...,An], where B is not an implication *)
fun strip_imp_prems (Const"==>" $ (Const"Trueprop" $ A) $ B) = 
           A :: strip_imp_prems B
  | strip_imp_prems (Const"==>" $ A $ B) = A :: strip_imp_prems B
  | strip_imp_prems _ = [];

(* A1==>...An==>B  goes to B, where B is not an implication *)
fun strip_imp_concl (Const"==>" $ A $ B) = strip_imp_concl B
  | strip_imp_concl (Const"Trueprop" $ A) = A
  | strip_imp_concl A = A : term;


(*** Conversion of Elimination Rules to Tableau Operations ***)

(*The conclusion becomes the goal/negated assumption *False*: delete it!*)
fun squash_nots [] = []
  | squash_nots (Const "*Goal*" $ (Var (ref (Some (Const"*False*")))) :: Ps) =
	squash_nots Ps
  | squash_nots (Const "Not" $ (Var (ref (Some (Const"*False*")))) :: Ps) =
	squash_nots Ps
  | squash_nots (P::Ps) = P :: squash_nots Ps;

fun skoPrem vars (Const "all" $ Abs (_, P)) =
        skoPrem vars (subst_bound (Skolem (gensym "S_", vars), P))
  | skoPrem vars P = P;

fun convertPrem t = 
    squash_nots (mkGoal (strip_imp_concl t) :: strip_imp_prems t);

(*Expects elimination rules to have a formula variable as conclusion*)
fun convertRule vars t =
  let val (P::Ps) = strip_imp_prems t
      val Var v   = strip_imp_concl t
  in  v := Some (Const"*False*");
      (P, map (convertPrem o skoPrem vars) Ps) 
  end;


(*Like dup_elim, but puts the duplicated major premise FIRST*)
fun rev_dup_elim th = th RSN (2, revcut_rl) |> assumption 2 |> Sequence.hd;


(*Count new hyps so that they can be rotated*)
fun nNewHyps []                         = 0
  | nNewHyps (Const "*Goal*" $ _ :: Ps) = nNewHyps Ps
  | nNewHyps (P::Ps)                    = 1 + nNewHyps Ps;

fun rot_subgoals_tac [] i st      = Sequence.single st
  | rot_subgoals_tac (k::ks) i st =
      rot_subgoals_tac ks (i+1) (Sequence.hd (rotate_tac (~k) i st))
      handle OPTION _ => Sequence.null;

fun TRACE rl tac st = if !trace then (prth rl; tac st) else tac st;

(*Tableau rule from elimination rule.  Flag "dup" requests duplication of the
  affected formula.*)
fun fromRule vars rl = 
  let val {tsig,...} = Sign.rep_sg (#sign (rep_thm rl))
      val trl = rl |> rep_thm |> #prop |> fromTerm tsig |> convertRule vars
      fun tac dup i = 
	  TRACE rl
	  (DETERM (etac (if dup then rev_dup_elim rl else rl) i))
	  THEN rot_subgoals_tac (map nNewHyps (#2 trl)) i
	  
  in General.SOME (trl, tac) end
  handle Bind => General.NONE  (*reject: conclusion is not just a variable*);


(*** Conversion of Introduction Rules (needed for efficiency in 
               proof reconstruction) ***)

fun convertIntrPrem t = mkGoal (strip_imp_concl t) :: strip_imp_prems t;

fun convertIntrRule vars t =
  let val Ps = strip_imp_prems t
      val P  = strip_imp_concl t
  in  (mkGoal P, map (convertIntrPrem o skoPrem vars) Ps) 
  end;

(*Tableau rule from introduction rule.  Since haz rules are now delayed, 
  "dup" is always FALSE for introduction rules.*)
fun fromIntrRule vars rl = 
  let val {tsig,...} = Sign.rep_sg (#sign (rep_thm rl))
      val trl = rl |> rep_thm |> #prop |> fromTerm tsig |> convertIntrRule vars
      fun tac dup i = 
	  TRACE rl (DETERM (rtac (if dup then Data.dup_intr rl else rl) i))
	  THEN rot_subgoals_tac (map nNewHyps (#2 trl)) i
  in (trl, tac) end;


val dummyVar = Term.Var (("Doom",666), dummyT);

(*Convert from prototerms to ordinary terms with dummy types
  Ignore abstractions; identify all Vars*)
fun dummyTerm 0 _             = dummyVar
  | dummyTerm d (Const a)     = Term.Const (a,dummyT)
  | dummyTerm d (OConst(a,_)) = Term.Const (a,dummyT)
  | dummyTerm d (Skolem(a,_)) = Term.Const (a,dummyT)
  | dummyTerm d (Free a)      = Term.Free  (a,dummyT)
  | dummyTerm d (Bound i)     = Term.Bound i
  | dummyTerm d (Var _)       = dummyVar
  | dummyTerm d (Abs(a,_))    = dummyVar
  | dummyTerm d (f $ u)       = Term.$ (dummyTerm d f, dummyTerm (d-1) u);


fun netMkRules P vars (nps: netpair list) =
  case P of
      (Const "*Goal*" $ G) =>
	let val pG = mk_tprop (dummyTerm 2 G)
	    val intrs = List.concat 
		             (map (fn (inet,_) => Net.unify_term inet pG) 
			      nps)
	in  map (fromIntrRule vars o #2) (orderlist intrs)  end
    | _ =>
	let val pP = mk_tprop (dummyTerm 3 P)
	    val elims = List.concat 
		             (map (fn (_,enet) => Net.unify_term enet pP) 
			      nps)
	in  List.mapPartial (fromRule vars o #2) (orderlist elims)  end;

(**
end;
**)

(*** Code for handling equality: naive substitution, like hyp_subst_tac ***)

(*Replace the ATOMIC term "old" by "new" in t*)  
fun subst_atomic (old,new) t =
    let fun subst (Var(ref(Some u))) = subst u
	  | subst (Abs(a,body))      = Abs(a, subst body)
	  | subst (f$t)              = subst f $ subst t
	  | subst t                  = if t aconv old then new else t
    in  subst t  end;

(*Eta-contract a term from outside: just enough to reduce it to an atom*)
fun eta_contract_atom (t0 as Abs(a, body)) = 
      (case  eta_contract2 body  of
        f $ Bound 0 => if (0 mem_int loose_bnos f) then t0
		       else eta_contract_atom (incr_boundvars ~1 f)
      | _ => t0)
  | eta_contract_atom t = t
and eta_contract2 (f$t) = f $ eta_contract_atom t
  | eta_contract2 t     = eta_contract_atom t;


(*When can we safely delete the equality?
    Not if it equates two constants; consider 0=1.
    Not if it resembles x=t[x], since substitution does not eliminate x.
    Not if it resembles ?x=0; another goal could instantiate ?x to Suc(i)
  Prefer to eliminate Bound variables if possible.
  Result:  true = use as is,  false = reorient first *)

(*Does t occur in u?  For substitution.  
  Does NOT check args of Skolem terms: substitution does not affect them.
  NOT reflexive since hyp_subst_tac fails on x=x.*)
fun substOccur t = 
  let fun occEq u = (t aconv u) orelse occ u
      and occ (Var(ref None))    = false
	| occ (Var(ref(Some u))) = occEq u
        | occ (Abs(_,u))         = occEq u
        | occ (f$u)              = occEq u  orelse  occEq f
        | occ (_)                = false;
  in  occEq  end;

fun check (t,u,v) = if substOccur t u then raise DEST_EQ else v;

(*IF the goal is an equality with a substitutable variable 
  THEN orient that equality ELSE raise exception DEST_EQ*)
fun orientGoal (t,u) =
  case (eta_contract_atom t, eta_contract_atom u) of
       (Skolem _, _) => check(t,u,(t,u))	(*eliminates t*)
     | (_, Skolem _) => check(u,t,(u,t))	(*eliminates u*)
     | (Free _, _)   => check(t,u,(t,u))	(*eliminates t*)
     | (_, Free _)   => check(u,t,(u,t))	(*eliminates u*)
     | _             => raise DEST_EQ;


(*Convert a Goal to an ordinary Not.  Used also in dup_intr, where a goal like
  Ex(P) is duplicated as the assumption ~Ex(P). *)
fun negOfGoal (Const"*Goal*" $ G, md) = (negate G, md)
  | negOfGoal G                   = G;

(*Substitute through the branch if an equality goal (else raise DEST_EQ)*)
fun equalSubst (G, br, hazs, lits, vars, lim) = 
  let val subst = subst_atomic (orientGoal(dest_eq G))
      fun subst2(G,md) = (subst G, md)
      fun subLits ([],        br, nlits) = 
	    (br, map (map subst2) hazs, nlits, vars, lim)
	| subLits (lit::lits, br, nlits) =
	    let val nlit = subst lit
	    in  if nlit aconv lit then subLits (lits, br, nlit::nlits)
		                  else subLits (lits, (nlit,true)::br, nlits)
            end
  in  subLits (rev lits,  map subst2 br,  [])  
  end;


exception NEWBRANCHES and CLOSEF;

type branch = (term*bool) list *	(*pending formulae with md flags*)
              (term*bool) list list *   (*stack of haz formulae*)
	      term list *               (*literals: irreducible formulae*)
	      term option ref list *    (*variables occurring in branch*)
	      int;                      (*resource limit*)

val fullTrace = ref[] : branch list list ref;

exception PROVE;

val eq_contr_tac = eresolve_tac [Data.notE]  THEN'  eq_assume_tac;

val eContr_tac  = TRACE Data.notE (eq_contr_tac ORELSE' Data.contr_tac);
val eAssume_tac = TRACE asm_rl   (eq_assume_tac ORELSE' assume_tac);

(*Try to unify complementary literals and return the corresponding tactic. *) 
fun tryClose (Const"*Goal*" $ G,  L) = (unify([],G,L); eAssume_tac)
  | tryClose (G,  Const"*Goal*" $ L) = (unify([],G,L); eAssume_tac)
  | tryClose (Const"Not" $ G,  L)    = (unify([],G,L); eContr_tac)
  | tryClose (G,  Const"Not" $ L)    = (unify([],G,L); eContr_tac)
  | tryClose _                       = raise UNIFY;


(*hazs is a list of lists of unsafe formulae.  This "stack" keeps them
  in the right relative order: they must go after *all* safe formulae, 
  with newly introduced ones coming before older ones.*)

(*Add an empty "stack frame" unless there's already one there*)
fun nilHaz hazs =
    case hazs of []::_ => hazs
               | _     => []::hazs;

fun addHaz (G, haz::hazs) = (haz@[negOfGoal G]) :: hazs;

(*Convert *Goal* to negated assumption in FIRST position*)
val negOfGoal_tac = rtac Data.ccontr THEN' rotate_tac ~1;

(*Were there Skolem terms in the premise?  Must NOT chase Vars*)
fun hasSkolem (Skolem _)     = true
  | hasSkolem (Abs (_,body)) = hasSkolem body 
  | hasSkolem (f$t)          =  hasSkolem f orelse hasSkolem t
  | hasSkolem _              = false;

(*Attach the right "may duplicate" flag to new formulae: if they contain
  Skolem terms then allow duplication.*)
fun joinMd md [] = []
  | joinMd md (G::Gs) = (G, hasSkolem G orelse md) :: joinMd md Gs;

(*Join new formulae to a branch.*)
fun appendBr md (ts,us) = 
    if (exists isGoal ts) then joinMd md ts @ map negOfGoal us
    else joinMd md ts @ us;

(** Backtracking and Pruning **)

(*clashVar vars (n,trail) determines whether any of the last n elements
  of "trail" occur in "vars" OR in their instantiations*)
fun clashVar [] = (fn _ => false)
  | clashVar vars =
      let fun clash (0, _)     = false
	    | clash (_, [])    = false
	    | clash (n, v::vs) = exists (varOccur v) vars orelse clash(n-1,vs)
      in  clash  end;


(*nbrs = # of branches just prior to closing this one.  Delete choice points
  for goals proved by the latest inference, provided NO variables in the
  next branch have been updated.*)
fun prune (1, nxtVars, choices) = choices  (*DON'T prune at very end: allow 
					     backtracking over bad proofs*)
  | prune (nbrs, nxtVars, choices) =
      let fun traceIt last =
		let val ll = length last
		    and lc = length choices
		in if !trace andalso ll<lc then
		    (writeln("PRUNING " ^ Int.toString(lc-ll) ^ " LEVELS"); 
		     last)
		   else last
		end
	  fun pruneAux (last, _, _, []) = last
	    | pruneAux (last, ntrl, trl, ch' as (ntrl',nbrs',exn) :: choices) =
		if nbrs' < nbrs 
		then last  (*don't backtrack beyond first solution of goal*)
		else if nbrs' > nbrs then pruneAux (last, ntrl, trl, choices)
		else (* nbrs'=nbrs *)
		     if clashVar nxtVars (ntrl-ntrl', trl) then last
		     else (*no clashes: can go back at least this far!*)
			  pruneAux(choices, ntrl', List.drop(trl, ntrl-ntrl'), 
				   choices)
  in  traceIt (pruneAux (choices, !ntrail, !trail, choices))  end;

fun nextVars ((br, hazs, lits, vars, lim) :: _) = map Var vars
  | nextVars []                                 = [];

fun backtrack ((_, _, exn)::_) = raise exn
  | backtrack _                = raise PROVE;

(*Change all *Goal* literals to Not.  Also delete all those identical to G.*)
fun addLit (Const "*Goal*" $ G,lits) = 
      let fun bad (Const"*Goal*" $ _) = true
	    | bad (Const"Not" $ G')   = G aconv G'
	    | bad _                   = false;
	  fun change [] = []
	    | change (Const"*Goal*" $ G' :: lits) = 
		  if G aconv G' then change lits
		  else Const"Not" $ G' :: change lits
	    | change (Const"Not" $ G' :: lits)    = 
		  if G aconv G' then change lits
		  else Const"Not" $ G' :: change lits
	    | change (lit::lits) = lit :: change lits
      in
	Const "*Goal*" $ G :: (if exists bad lits then change lits else lits)
      end
  | addLit (G,lits) = ins_term(G, lits)


(*Tableau prover based on leanTaP.  Argument is a list of branches.  Each 
  branch contains a list of unexpanded formulae, a list of literals, and a 
  bound on unsafe expansions.*)
fun prove (cs, brs, cont) =
 let val {safe0_netpair, safep_netpair, haz_netpair, ...} = Data.rep_claset cs
     val safeList = [safe0_netpair, safep_netpair]
     and hazList  = [haz_netpair]
     fun prv (tacs, trs, choices, []) = (cont (trs,choices,tacs))
       | prv (tacs, trs, choices, 
	      brs0 as ((G,md)::br, hazs, lits, vars, lim) :: brs) =
	  let exception PRV (*backtrack to precisely this recursion!*)
	      val ntrl = !ntrail 
	      val nbrs = length brs0
              val nxtVars = nextVars brs
	      val G = norm G
	      (*Make a new branch, decrementing "lim" if instantiations occur*)
	      fun newBr vars prems =
		  map (fn prem => (appendBr md (prem, br),  
				   nilHaz hazs,  lits,
				   add_terms_vars (prem,vars), 
				   if ntrl < !ntrail then lim-3 else lim)) 
		  prems @
		  brs		  
	      (*Seek a matching rule.  If unifiable then add new premises
                to branch.*)
	      fun deeper [] = raise NEWBRANCHES
		| deeper (((P,prems),tac)::grls) =
		    let val dummy = unify(add_term_vars(P,[]), P, G)
			    (*P comes from the rule; G comes from the branch.*)
                        val ntrl' = !ntrail
			val choices' = (ntrl, nbrs, PRV) :: choices
                    in
			if null prems then (*closed the branch: prune!*)
			  prv(tac false :: tacs,	(*no duplication*)
			      brs0::trs, 
			      prune (nbrs, nxtVars, choices'),
			      brs)
			  handle PRV => 
			      (*reset Vars and try another rule*)
			      (clearTo ntrl;  deeper grls)
		        else 
			  prv(tac false :: tacs,	(*no duplication*)
			      brs0::trs, choices',
			      newBr (vars_in_vars vars) prems)
			  handle PRV => 
			      if ntrl < ntrl' then
				   (*Vars have been updated: must backtrack
				     even if not mentioned in other goals!
				     Reset Vars and try another rule*)
				   (clearTo ntrl;  deeper grls)
			      else (*backtrack to previous level*)
				   backtrack choices
		    end
		    handle UNIFY => deeper grls
	      (*Try to close branch by unifying with head goal*)
	      fun closeF [] = raise CLOSEF
		| closeF (L::Ls) = 
		    let val tacs' = tryClose(G,L)::tacs
			val choices' = prune (nbrs, nxtVars, 
					      (ntrl, nbrs, PRV) :: choices)
		    in  prv (tacs', brs0::trs, choices', brs)
			handle PRV => 
			    (*reset Vars and try another literal
			      [this handler is pruned if possible!]*)
			 (clearTo ntrl;  closeF Ls)
                    end
		    handle UNIFY => closeF Ls
	  in if !trace then fullTrace := brs0 :: !fullTrace else ();
	     if lim<0 then backtrack choices
	     else
	     prv (Data.vars_gen_hyp_subst_tac false :: tacs, 
		  brs0::trs,  choices,
		  equalSubst (G, br, hazs, lits, vars, lim) :: brs)
	     handle DEST_EQ => closeF lits
	      handle CLOSEF => closeF (map #1 br)
	       handle CLOSEF => closeF (map #1 (List.concat hazs))
	        handle CLOSEF => 
		   (deeper (netMkRules G vars safeList)
		    handle NEWBRANCHES => 
		     (case netMkRules G vars hazList of
			 [] => (*no plausible rules: move G to literals*)
			     prv (tacs, trs, choices,
				  (br, hazs, addLit(G,lits), vars, lim)::brs)
		      | _ => (*G admits some haz rules: try later*)
			     prv (if isGoal G then negOfGoal_tac :: tacs
				  else tacs, 
				  trs,  choices,
				  (br, addHaz((G,md),hazs), lits, vars, lim)
				  ::brs)))
	  end
       | prv (tacs, trs, choices, ([], []::hazs, lits, vars, lim) :: brs) =
			(*removal of empty list from hazs*)
	   prv (tacs, trs, choices, ([], hazs, lits, vars, lim) :: brs)
       | prv (tacs, trs, choices, 
	      brs0 as ([], ((G,md)::Gs)::hazs, lits, vars, lim) :: brs) =
			(*application of haz rule*)
	  let exception PRV (*backtrack to precisely this recursion!*)
	      val G = norm G
	      val ntrl = !ntrail
	      fun newPrem (vars,dup) prem = 
		  (map (fn P => (P,false)) prem,   
		   nilHaz (if dup then Gs :: hazs @ [[negOfGoal (G,md)]]
			   else Gs :: hazs),  
		   lits,  
		   vars,  
		   (*Decrement "lim" if instantiations occur or the
		     formula is duplicated*)
		   if ntrl < !ntrail then lim-3 
		   else if dup then lim-1 else lim)
	      fun newBr x prems = map (newPrem x) prems  @  brs
	      (*Seek a matching rule.  If unifiable then add new premises
                to branch.*)
	      fun deeper [] = raise NEWBRANCHES
		| deeper (((P,prems),tac)::grls) =
		    let val dummy = unify(add_term_vars(P,[]), P, G)
			val ntrl' = !ntrail
                        val vars  = vars_in_vars vars
                        val vars' = foldr add_terms_vars (prems, vars)
                        val dup = md andalso vars' <> vars
			(*duplicate G only if md and the premise has new vars*)
                    in
		      prv(tac dup :: tacs, 
			  brs0::trs, 
			  (ntrl, length brs0, PRV) :: choices, 
			  newBr (vars', dup) prems)
		     handle PRV => 
			 if ntrl < ntrl'       (*variables updated?*)
			    orelse vars=vars'  (*pseudo-unsafe: no new Vars?*)
			 then (*reset Vars and try another rule*)
			      (clearTo ntrl;  deeper grls)
			 else (*backtrack to previous level*)
			      backtrack choices
		    end
		    handle UNIFY => deeper grls
	  in if !trace then fullTrace := brs0 :: !fullTrace else ();
	     if lim<1 then backtrack choices
	     else
	     deeper (netMkRules G vars hazList)
	     handle NEWBRANCHES => 
		 (*cannot close branch: move G to literals*)
		 prv (tacs,  brs0::trs,  choices,
		      ([], Gs::hazs, G::lits, vars, lim)::brs)
	  end
       | prv (tacs, trs, choices, _ :: brs) = backtrack choices
 in prv ([], [], [(!ntrail, length brs, PROVE)], brs) end;


fun initBranch (ts,lim) = 
    (map (fn t => (t,true)) ts, 
     [[]], [], add_terms_vars (ts,[]), lim);


(*** Conversion & Skolemization of the Isabelle proof state ***)

(*Make a list of all the parameters in a subgoal, even if nested*)
local open Term 
in
fun discard_foralls (Const("all",_)$Abs(a,T,t)) = discard_foralls t
  | discard_foralls t = t;
end;


(*List of variables not appearing as arguments to the given parameter*)
fun getVars []                  i = []
  | getVars ((_,(v,is))::alist) i =
	if i mem is then getVars alist i
	else v :: getVars alist i;


(*Conversion of a subgoal: Skolemize all parameters*)
fun fromSubgoal tsig t =
  let val alist = ref []
      fun hdvar ((ix,(v,is))::_) = v
      fun from lev t =
	let val (ht,ts) = Term.strip_comb t
	    fun apply u = list_comb (u, map (from lev) ts)
	    fun bounds [] = []
	      | bounds (Term.Bound i::ts) = 
		  if i<lev then error"Function Var's argument not a parameter"
		  else i-lev :: bounds ts
	      | bounds ts = error"Function Var's argument not a bound variable"
        in
	  case ht of 
	      t as Term.Const _ => apply (fromConst tsig t)	    
	    | Term.Free  (a,_) => apply (Free a)
	    | Term.Bound i     => apply (Bound i)
	    | Term.Var (ix,_) => 
		  (case (assoc_string_int(!alist,ix)) of
		       None => (alist := (ix, (ref None, bounds ts))
					  :: !alist;
				Var (hdvar(!alist)))
		     | Some(v,is) => if is=bounds ts then Var v
			    else error("Discrepancy among occurrences of ?"
				       ^ Syntax.string_of_vname ix))
	    | Term.Abs (a,_,body) => 
		  if null ts then Abs(a, from (lev+1) body)
		  else error "fromSubgoal: argument not in normal form"
        end

      val npars = length (Logic.strip_params t)

      (*Skolemize a subgoal from a proof state*)
      fun skoSubgoal i t =
	  if i<npars then 
	      skoSubgoal (i+1)
		(subst_bound (Skolem (gensym "SG_", getVars (!alist) i), 
			      t))
	  else t

  in  skoSubgoal 0 (from 0 (discard_foralls t))  end;


(*Tactic using tableau engine and proof reconstruction.  
 "lim" is depth limit.*)
fun depth_tac cs lim i st = 
    (fullTrace:=[];  trail := [];  ntrail := 0;
     let val {tsig,...} = Sign.rep_sg (#sign (rep_thm st))
	 val skoprem = fromSubgoal tsig (List.nth(prems_of st, i-1))
         val hyps  = strip_imp_prems skoprem
         and concl = strip_imp_concl skoprem
         fun cont (_,choices,tacs) = 
	     (case Sequence.pull(EVERY' (rev tacs) i st) of
		  None => (writeln ("PROOF FAILED for depth " ^
				    Int.toString lim);
			   backtrack choices)
		| cell => Sequence.seqof(fn()=> cell))
     in prove (cs, [initBranch (mkGoal concl :: hyps, lim)], cont)
     end
     handle Subscript => Sequence.null
	  | PROVE     => Sequence.null);

fun blast_tac cs = (DEEPEN (1,20) (depth_tac cs) 0);

fun Blast_tac i = blast_tac (!Data.claset) i;

end;