src/HOL/Tools/Sledgehammer/clausifier.ML
author blanchet
Thu Sep 09 14:47:06 2010 +0200 (2010-09-09)
changeset 39261 b1bfb3de88fd
parent 39198 f967a16dfcdd
child 39268 a56f931fffff
permissions -rw-r--r--
add cutoff beyond which facts are handled using definitional CNF
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(*  Title:      HOL/Tools/Sledgehammer/clausifier.ML
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    Author:     Jia Meng, Cambridge University Computer Laboratory and NICTA
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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 min_clauses_for_definitional_cnf : int
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  val extensionalize_theorem : thm -> thm
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  val introduce_combinators_in_cterm : cterm -> thm
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  val introduce_combinators_in_theorem : thm -> thm
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  val to_definitional_cnf_with_quantifiers : theory -> thm -> thm
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  val cnf_axiom : theory -> thm -> thm list
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end;
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structure Clausifier : CLAUSIFIER =
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struct
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(* Cutoff beyond which definitional CNF is used. Definitional CNF has a certain
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   overhead, but it prevents the exponential explosion of the number of
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   clauses. *)
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val min_clauses_for_definitional_cnf = 30
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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
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   the conclusion variable to False. (Cf. "transform_elim_term" in
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   "Sledgehammer_Util".) *)
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fun transform_elim_theorem 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
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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 HOL.conj} $ p $ q) rhss = rhss |> dec_sko p |> dec_sko q
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      | dec_sko (@{const HOL.disj} $ 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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val fun_cong_all = @{thm ext_iff [THEN iffD1]}
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(* Removes the lambdas from an equation of the form "t = (%x. u)".
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   (Cf. "extensionalize_term" in "Sledgehammer_Translate".) *)
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fun extensionalize_theorem th =
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  case prop_of th of
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    _ $ (Const (@{const_name HOL.eq}, Type (_, [Type (@{type_name fun}, _), _]))
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         $ _ $ Abs (s, _, _)) => extensionalize_theorem (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
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      val cbodyT = ctyp_of thy bodyT
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      fun makeK () =
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        instantiate' [SOME cxT, SOME cbodyT] [SOME (cterm_of thy body)]
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                     @{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 introduce_combinators_in_cterm 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 = introduce_combinators_in_cterm 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 (introduce_combinators_in_cterm ct1)
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                        (introduce_combinators_in_cterm ct2)
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    end
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fun introduce_combinators_in_theorem 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 = introduce_combinators_in_cterm (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 = 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 rhs0 =
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  let
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    val rhs = rhs0 |> Type.legacy_freeze_thaw |> #1 |> 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 = 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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      rewrite_goals_tac skolem_id_def_raw
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      THEN rtac ((prem |> rewrite_rule skolem_id_def_raw) 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)
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   into NNF. *)
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fun to_nnf th ctxt0 =
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  let
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    val th1 = th |> transform_elim_theorem |> 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_theorem
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                  |> Meson.make_nnf ctxt
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  in (th3, ctxt) end
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fun to_definitional_cnf_with_quantifiers thy th =
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  let
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    val eqth = cnf.make_cnfx_thm thy (HOLogic.dest_Trueprop (prop_of th))
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    val eqth = eqth RS @{thm eq_reflection}
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    val eqth = eqth RS @{thm TruepropI}
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  in Thm.equal_elim eqth th end
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(* Convert a theorem to CNF, with Skolem functions as additional premises. *)
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fun cnf_axiom thy th =
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  let
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    val ctxt0 = Variable.global_thm_context th
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    val (nnf_th, ctxt) = to_nnf th ctxt0
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    fun aux th =
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      Meson.make_cnf (map (skolem_theorem_of_def thy) (assume_skolem_funs th))
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                     th ctxt
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    val (cnf_ths, ctxt) =
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      aux nnf_th
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      |> (fn (cnf_ths, ctxt) =>
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             if null cnf_ths orelse
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                length cnf_ths >= min_clauses_for_definitional_cnf then
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               aux (to_definitional_cnf_with_quantifiers thy nnf_th)
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             else
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               (cnf_ths, ctxt))
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  in
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    cnf_ths |> map introduce_combinators_in_theorem
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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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end;