unfold INC in |- *; intros.
apply INC_IN_Power.
cut (INC E0 E).
unfold INC in |- *; unfold INC in H; intros; auto with zfc.
apply IN_Power_INC; auto with zfc.
Qed.

Theorem Power_sound : forall E E' : Ens, EQ E E' -> EQ (Power E) (Power E').
intros E E' e.