(* Proof of lem_Vide_rel_sig                                               *)
exact (lem_vide_is_inc (cartesien a a)).
Qed.
(* End of proof of lem_Vide_rel_sig                                        *)

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(* Le domaine de la relation vide est l'ensemble vide                      *)
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Lemma lem_Vide_dom : forall a : E, rdom Vide a a (lem_Vide_rel_sig a) = Vide.
intros; apply (lem_x_inc_vide (rdom Vide a a (lem_Vide_rel_sig a)));
unfold inc in |- *; intros.