unfold inclus in |- *.
intros.
cut
(exists x1 : Elt,
(exists x2 : Elt, dans x1 a /\ dans x2 c /\ x = couple x1 x2)).
2: apply coupl3; auto.
intro H2; elim H2; clear H2.
intros x1 H2; elim H2; clear H2.
intros x2 H2; elim H2; clear H2.
intros H2 H3; elim H3; clear H3.
intros H3 H4.
rewrite H4.
auto.
Qed.
Hint Resolve cart_inclus.

(* Specifications: *)
Lemma inclus_add :
forall (a b : Ensf) (y : Elt), inclus a b -> inclus a (add y b).