intros.
cut (Inmonoid X (cons a w)); auto.
intro.
cut (dans a X /\ Inmonoid X w); auto.
intro H1; elim H1; clear H1.
auto.
Qed.

(* Specifications: *)
Lemma inmonoid_inclus :
forall (E F : Ensf) (x : Word), inclus E F -> inmonoid E x -> inmonoid F x.