intros.
elim H; [ unfold Word_ext in |- *; auto | idtac ].
intros; unfold Word_ext in |- *; simpl in |- *.
apply inmonoid_cons; try apply dans_map_inv; auto.
Qed.
Hint Resolve inmonoid_map.
(*  Un petit lemme bien utile par la suite...				*)

(* Specifications: *)
Lemma cons_cons :
forall (x1 x2 : Elt) (w1 w2 : Word),
x1 = x2 :>Elt -> w1 = w2 :>Word -> cons x1 w1 = cons x2 w2 :>Word.