unfold inclus in |- *.
intros e a x H.
cut (x = e); auto.
intro H0.
rewrite H0; auto.
Qed.
Hint Resolve singl_inclus_add.

(* Specifications: *)
Lemma inclus_singl :
forall (e : Elt) (a : Ensf), inclus (singleton e) a -> dans e a.