src/HOL/Tools/Sledgehammer/clausifier.ML
author blanchet
Tue Jun 29 11:03:26 2010 +0200 (2010-06-29)
changeset 37629 a4f129820562
parent 37628 78334f400ae6
child 37995 06f02b15ef8a
permissions -rw-r--r--
more elegant cheating
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(*  Title:      HOL/Tools/Sledgehammer/clausifier.ML
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    Author:     Jia Meng, Cambridge University Computer Laboratory
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    Author:     Jasmin Blanchette, TU Muenchen
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Transformation of axiom rules (elim/intro/etc) into CNF forms.
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*)
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signature CLAUSIFIER =
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sig
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  val cnf_axiom: theory -> bool -> thm -> thm list
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  val cnf_rules_pairs :
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    theory -> bool -> (string * thm) list -> ((string * int) * thm) list
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  val neg_clausify: thm -> thm list
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  val neg_conjecture_clauses:
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    Proof.context -> thm -> int -> thm list list * (string * typ) list
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end;
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structure Clausifier : CLAUSIFIER =
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struct
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(**** Transformation of Elimination Rules into First-Order Formulas****)
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val cfalse = cterm_of @{theory HOL} HOLogic.false_const;
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val ctp_false = cterm_of @{theory HOL} (HOLogic.mk_Trueprop HOLogic.false_const);
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(*Converts an elim-rule into an equivalent theorem that does not have the
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  predicate variable.  Leaves other theorems unchanged.  We simply instantiate the
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  conclusion variable to False.*)
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fun transform_elim th =
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  case concl_of th of    (*conclusion variable*)
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       @{const Trueprop} $ (v as Var (_, @{typ bool})) =>
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           Thm.instantiate ([], [(cterm_of @{theory HOL} v, cfalse)]) th
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    | v as Var(_, @{typ prop}) =>
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           Thm.instantiate ([], [(cterm_of @{theory HOL} v, ctp_false)]) th
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    | _ => th;
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(*To enforce single-threading*)
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exception Clausify_failure of theory;
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(**** SKOLEMIZATION BY INFERENCE (lcp) ****)
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fun mk_skolem_id t =
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  let val T = fastype_of t in
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    Const (@{const_name skolem_id}, T --> T) $ t
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  end
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fun beta_eta_under_lambdas (Abs (s, T, t')) =
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    Abs (s, T, beta_eta_under_lambdas t')
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  | beta_eta_under_lambdas t = Envir.beta_eta_contract t
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(*Traverse a theorem, accumulating Skolem function definitions.*)
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fun assume_skolem_funs th =
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  let
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    fun dec_sko (Const (@{const_name Ex}, _) $ (body as Abs (s', T, p))) rhss =
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        (*Existential: declare a Skolem function, then insert into body and continue*)
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        let
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          val args = OldTerm.term_frees body
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          val Ts = map type_of args
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          val cT = Ts ---> T (* FIXME: use "skolem_type_and_args" *)
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          (* Forms a lambda-abstraction over the formal parameters *)
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          val rhs =
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            list_abs_free (map dest_Free args,
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                           HOLogic.choice_const T $ beta_eta_under_lambdas body)
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            |> mk_skolem_id
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          val comb = list_comb (rhs, args)
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        in dec_sko (subst_bound (comb, p)) (rhs :: rhss) end
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      | dec_sko (Const (@{const_name All},_) $ Abs (a, T, p)) rhss =
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        (*Universal quant: insert a free variable into body and continue*)
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        let val fname = Name.variant (OldTerm.add_term_names (p,[])) a
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        in dec_sko (subst_bound (Free(fname,T), p)) rhss end
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      | dec_sko (@{const "op &"} $ p $ q) rhss = rhss |> dec_sko p |> dec_sko q
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      | dec_sko (@{const "op |"} $ p $ q) rhss = rhss |> dec_sko p |> dec_sko q
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      | dec_sko (@{const Trueprop} $ p) rhss = dec_sko p rhss
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      | dec_sko _ rhss = rhss
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  in  dec_sko (prop_of th) []  end;
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(**** REPLACING ABSTRACTIONS BY COMBINATORS ****)
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(*Returns the vars of a theorem*)
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fun vars_of_thm th =
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  map (Thm.cterm_of (theory_of_thm th) o Var) (Thm.fold_terms Term.add_vars th []);
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val fun_cong_all = @{thm expand_fun_eq [THEN iffD1]}
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(* Removes the lambdas from an equation of the form t = (%x. u). *)
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fun extensionalize th =
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  case prop_of th of
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    _ $ (Const (@{const_name "op ="}, Type (_, [Type (@{type_name fun}, _), _]))
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         $ _ $ Abs (s, _, _)) => extensionalize (th RS fun_cong_all)
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  | _ => th
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fun is_quasi_lambda_free (Const (@{const_name skolem_id}, _) $ _) = true
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  | is_quasi_lambda_free (t1 $ t2) =
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    is_quasi_lambda_free t1 andalso is_quasi_lambda_free t2
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  | is_quasi_lambda_free (Abs _) = false
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  | is_quasi_lambda_free _ = true
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val [f_B,g_B] = map (cterm_of @{theory}) (OldTerm.term_vars (prop_of @{thm abs_B}));
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val [g_C,f_C] = map (cterm_of @{theory}) (OldTerm.term_vars (prop_of @{thm abs_C}));
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val [f_S,g_S] = map (cterm_of @{theory}) (OldTerm.term_vars (prop_of @{thm abs_S}));
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(*FIXME: requires more use of cterm constructors*)
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fun abstract ct =
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  let
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      val thy = theory_of_cterm ct
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      val Abs(x,_,body) = term_of ct
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      val Type(@{type_name fun}, [xT,bodyT]) = typ_of (ctyp_of_term ct)
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      val cxT = ctyp_of thy xT and cbodyT = ctyp_of thy bodyT
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      fun makeK() = instantiate' [SOME cxT, SOME cbodyT] [SOME (cterm_of thy body)] @{thm abs_K}
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  in
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      case body of
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          Const _ => makeK()
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        | Free _ => makeK()
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        | Var _ => makeK()  (*though Var isn't expected*)
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        | Bound 0 => instantiate' [SOME cxT] [] @{thm abs_I} (*identity: I*)
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        | rator$rand =>
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            if loose_bvar1 (rator,0) then (*C or S*)
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               if loose_bvar1 (rand,0) then (*S*)
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                 let val crator = cterm_of thy (Abs(x,xT,rator))
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                     val crand = cterm_of thy (Abs(x,xT,rand))
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                     val abs_S' = cterm_instantiate [(f_S,crator),(g_S,crand)] @{thm abs_S}
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                     val (_,rhs) = Thm.dest_equals (cprop_of abs_S')
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                 in
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                   Thm.transitive abs_S' (Conv.binop_conv abstract rhs)
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                 end
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               else (*C*)
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                 let val crator = cterm_of thy (Abs(x,xT,rator))
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                     val abs_C' = cterm_instantiate [(f_C,crator),(g_C,cterm_of thy rand)] @{thm abs_C}
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                     val (_,rhs) = Thm.dest_equals (cprop_of abs_C')
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                 in
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                   Thm.transitive abs_C' (Conv.fun_conv (Conv.arg_conv abstract) rhs)
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                 end
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            else if loose_bvar1 (rand,0) then (*B or eta*)
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               if rand = Bound 0 then Thm.eta_conversion ct
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               else (*B*)
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                 let val crand = cterm_of thy (Abs(x,xT,rand))
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                     val crator = cterm_of thy rator
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                     val abs_B' = cterm_instantiate [(f_B,crator),(g_B,crand)] @{thm abs_B}
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                     val (_,rhs) = Thm.dest_equals (cprop_of abs_B')
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                 in Thm.transitive abs_B' (Conv.arg_conv abstract rhs) end
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            else makeK()
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        | _ => raise Fail "abstract: Bad term"
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  end;
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(* Traverse a theorem, remplacing lambda-abstractions with combinators. *)
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fun do_introduce_combinators ct =
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  if is_quasi_lambda_free (term_of ct) then
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    Thm.reflexive ct
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  else case term_of ct of
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    Abs _ =>
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    let
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      val (cv, cta) = Thm.dest_abs NONE ct
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      val (v, _) = dest_Free (term_of cv)
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      val u_th = do_introduce_combinators cta
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      val cu = Thm.rhs_of u_th
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      val comb_eq = abstract (Thm.cabs cv cu)
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    in Thm.transitive (Thm.abstract_rule v cv u_th) comb_eq end
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  | _ $ _ =>
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    let val (ct1, ct2) = Thm.dest_comb ct in
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        Thm.combination (do_introduce_combinators ct1)
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                        (do_introduce_combinators ct2)
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    end
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fun introduce_combinators th =
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  if is_quasi_lambda_free (prop_of th) then
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    th
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  else
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    let
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      val th = Drule.eta_contraction_rule th
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      val eqth = do_introduce_combinators (cprop_of th)
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    in Thm.equal_elim eqth th end
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    handle THM (msg, _, _) =>
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           (warning ("Error in the combinator translation of " ^
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                     Display.string_of_thm_without_context th ^
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                     "\nException message: " ^ msg ^ ".");
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            (* A type variable of sort "{}" will make abstraction fail. *)
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            TrueI)
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(*cterms are used throughout for efficiency*)
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val cTrueprop = Thm.cterm_of @{theory HOL} HOLogic.Trueprop;
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(*Given an abstraction over n variables, replace the bound variables by free
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  ones. Return the body, along with the list of free variables.*)
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fun c_variant_abs_multi (ct0, vars) =
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      let val (cv,ct) = Thm.dest_abs NONE ct0
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      in  c_variant_abs_multi (ct, cv::vars)  end
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      handle CTERM _ => (ct0, rev vars);
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val skolem_id_def_raw = @{thms skolem_id_def_raw}
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(* Given the definition of a Skolem function, return a theorem to replace
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   an existential formula by a use of that function.
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   Example: "EX x. x : A & x ~: B ==> sko A B : A & sko A B ~: B"  [.] *)
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fun skolem_theorem_of_def thy cheat rhs0 =
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  let
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    val rhs = rhs0 |> Type.legacy_freeze_thaw |> #1 |> Thm.cterm_of thy
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    val rhs' = rhs |> Thm.dest_comb |> snd
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    val (ch, frees) = c_variant_abs_multi (rhs', [])
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    val (hilbert, cabs) = ch |> Thm.dest_comb |>> term_of
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    val T =
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      case hilbert of
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        Const (@{const_name Eps}, Type (@{type_name fun}, [_, T])) => T
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      | _ => raise TERM ("skolem_theorem_of_def: expected \"Eps\"", [hilbert])
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    val cex = Thm.cterm_of thy (HOLogic.exists_const T)
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    val ex_tm = Thm.capply cTrueprop (Thm.capply cex cabs)
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    val conc =
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      Drule.list_comb (rhs, frees)
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      |> Drule.beta_conv cabs |> Thm.capply cTrueprop
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    fun tacf [prem] =
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      if cheat then
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        Skip_Proof.cheat_tac thy
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      else
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        rewrite_goals_tac skolem_id_def_raw
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        THEN rtac ((prem |> rewrite_rule skolem_id_def_raw)
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                   RS @{thm someI_ex}) 1
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  in
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    Goal.prove_internal [ex_tm] conc tacf
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    |> forall_intr_list frees
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    |> Thm.forall_elim_vars 0  (*Introduce Vars, but don't discharge defs.*)
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    |> Thm.varifyT_global
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  end
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(*Converts an Isabelle theorem (intro, elim or simp format, even higher-order) into NNF.*)
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fun to_nnf th ctxt0 =
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  let val th1 = th |> transform_elim |> zero_var_indexes
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      val ((_, [th2]), ctxt) = Variable.import true [th1] ctxt0
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      val th3 = th2 |> Conv.fconv_rule Object_Logic.atomize
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                    |> extensionalize
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                    |> Meson.make_nnf ctxt
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  in  (th3, ctxt)  end;
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(*Skolemize a named theorem, with Skolem functions as additional premises.*)
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fun skolemize_theorem thy cheat th =
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  let
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    val ctxt0 = Variable.global_thm_context th
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    val (nnfth, ctxt) = to_nnf th ctxt0
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    val sko_ths = map (skolem_theorem_of_def thy cheat)
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                      (assume_skolem_funs nnfth)
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    val (cnfs, ctxt) = Meson.make_cnf sko_ths nnfth ctxt
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  in
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    cnfs |> map introduce_combinators
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         |> Variable.export ctxt ctxt0
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         |> Meson.finish_cnf
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         |> map Thm.close_derivation
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  end
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  handle THM _ => []
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(* Convert Isabelle theorems into axiom clauses. *)
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(* FIXME: is transfer necessary? *)
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fun cnf_axiom thy cheat = skolemize_theorem thy cheat o Thm.transfer thy
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(**** Translate a set of theorems into CNF ****)
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(*The combination of rev and tail recursion preserves the original order*)
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fun cnf_rules_pairs thy cheat =
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  let
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    fun do_one _ [] = []
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      | do_one ((name, k), th) (cls :: clss) =
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        ((name, k), cls) :: do_one ((name, k + 1), th) clss
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    fun do_all pairs [] = pairs
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      | do_all pairs ((name, th) :: ths) =
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        let
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          val new_pairs = do_one ((name, 0), th) (cnf_axiom thy cheat th)
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                          handle THM _ => []
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        in do_all (new_pairs @ pairs) ths end
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  in do_all [] o rev end
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(*** Converting a subgoal into negated conjecture clauses. ***)
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fun neg_skolemize_tac ctxt =
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  EVERY' [rtac ccontr, Object_Logic.atomize_prems_tac, Meson.skolemize_tac ctxt]
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val neg_clausify =
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  single
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  #> Meson.make_clauses_unsorted
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  #> map introduce_combinators
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  #> Meson.finish_cnf
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fun neg_conjecture_clauses ctxt st0 n =
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  let
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    (* "Option" is thrown if the assumptions contain schematic variables. *)
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    val st = Seq.hd (neg_skolemize_tac ctxt n st0) handle Option.Option => st0
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    val ({params, prems, ...}, _) =
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      Subgoal.focus (Variable.set_body false ctxt) n st
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  in (map neg_clausify prems, map (dest_Free o term_of o #2) params) end
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end;