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(* 02110-1301 USA                                                     *)
(* Contribution to the Coq Library   V6.3 (July 1999)                    *)
(****************************************************************************)
(*                 The Calculus of Inductive Constructions                  *)
(*                                                                          *)
(*                                Projet Coq                                *)
(*                                                                          *)
(*                     INRIA                        ENS-CNRS                *)
(*              Rocquencourt                        Lyon                    *)
(*                                                                          *)
(*                                Coq V5.10                                 *)
(*                              Nov 25th 1994                               *)
(*                                                                          *)
(****************************************************************************)
(*                               Ensf_types.v                               *)
(****************************************************************************)
(*              Formal Language Theory                                      *)
(*                                                                          *)
(*              Judicael Courant - Jean-Christophe Filliatre                *)
(*                                                                          *)
(*              Developped in V5.8  June-July 1993                          *)
(*              Ported     to V5.10 October 1994                            *)
(****************************************************************************)
(*  On definit 3 "types" mutuellement inductifs : Elt, Ensf et Word      *)
(*  On distingue elt et mot, car on a besoin du type mot plus tard.      *)
(*  Les constructeurs up et word permettent repectivement de considerer  *)
(*  un ensemble ou un mot en tant qu'element.				 *)
Inductive Ensf : Set :=
| empty : Ensf
| add : Elt -> Ensf -> Ensf
with Elt : Set :=
| natural : nat -> Elt
| couple : Elt -> Elt -> Elt
| up : Ensf -> Elt
| word : Word -> Elt
with Word : Set :=
| nil : Word
| cons : Elt -> Word -> Word.
(*  Inversion de quelques constructeurs...  *)
(* Definition natural_inv : Elt -> nat := [e:Elt] (<nat>Case e of (* natural *) [n:nat]n (* couple  *) [a:Elt][b:Elt]O (* up      *) [e:Ensf]O (* word    *) [w:Word]O end ). *)
Definition natural_inv (e : Elt) : nat :=
match e with
| natural n => n
| _ => 0
end.

Lemma nat_invol : forall n : nat, natural_inv (natural n) = n.
auto.