(* File: Normal_Form.v  (last edited on 27/10/2000) (c) Klaus Weich  *)
Require Export Forms.
(*******   Normal forms    ***********************************************)
Inductive normal_form : Set :=
| NFalsum : normal_form
| NAtom : Int -> normal_form
| NDisj : Int -> Int -> normal_form
| AImp : Int -> normal_form -> normal_form
| NImp_NF : nimp -> normal_form
with nimp : Set :=
NImp : Int -> Int -> normal_form -> nimp.
Fixpoint nf2form (x : normal_form) : form :=
match x with
| NFalsum => Falsum
| NAtom i => Atom i
| NDisj i j => OrF (Atom i) (Atom j)
| AImp i b => Imp (Atom i) (nf2form b)
| NImp_NF x => nimp2form x
end
with nimp2form (x : nimp) : form :=
match x with
| NImp i j b => Imp (Imp (Atom i) (Atom j)) (nf2form b)
end.
Definition nf_list := list normal_form.
Definition nf_nil := nil (A:=normal_form).
Fixpoint nvimp (l : list Int) : normal_form -> normal_form :=
match l with
| nil => fun a : normal_form => a
| i :: l => fun a : normal_form => nvimp l (AImp i a)
end.
intros l; elim l; clear l.
intros a; exists a; trivial.
intros i l ih a.
elim (ih (AImp i a)); clear ih.
intros b nf_b.
exists b; assumption.
Qed.

Lemma vimp2nvimp :
forall (l : list Int) (a : normal_form), {b : normal_form | b = nvimp l a}.