proposition: Lemma map_union : forall (f : Elt -> Elt) (a b : Ensf), union (map f a) (map f b) = map f (union a b) :>Ensf.
proof: Proof. intro f. simple induction a; simpl in |- *; auto. Qed.
imports: Ensf_types Ensf_dans Ensf_union Ensf_inclus