unfold Vide in |- *; simple induction E; simpl in |- *; intros A e X H;
split.
intros; elim (H (e x)); auto with zfc.
exists x; auto with zfc.
simple induction y.
Qed.

Theorem Paire_sound_leftC :
forall A A' B : Ens, EQC A A' -> EQC (Paire A B) (Paire A' B).