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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_union.v                               *)
(****************************************************************************)
(*              Formal Language Theory                                      *)
(*                                                                          *)
(*              Judicael Courant - Jean-Christophe Filliatre                *)
(*                                                                          *)
(*              Developped in V5.8  June-July 1993                          *)
(*              Ported     to V5.10 October 1994                            *)
(****************************************************************************)
(*									*)
(*  UNION :								*)
(*    On definit ici l'union de 2 ensembles. On remarquera qu'un 	*)
(*    peut apparaitre plusieurs fois dans un ensemble, et que c'est     *)
(*    pour cela que l'on utilise 					*)
(*      union E (add x F) -> add x (union E F)				*)
(*    directement.							*)
(*									*)
Fixpoint union (A : Ensf) : Ensf -> Ensf :=
fun B : Ensf =>
match A return Ensf with
| empty =>
(* empty *)  B (* add   *)
| add x e => add x (union e B)
end.
simple induction a.
apply refl_equal.
intros a0 b H.
simpl in |- *; auto.
Qed.
Hint Resolve union_a_empty.

Lemma dans_union :
forall (x : Elt) (a b : Ensf), dans x (union a b) -> dans x a \/ dans x b.