elim (IN_Vee_EXType _ _ i).
intros Y; simple induction 1; intros i1 inc1.
apply IN_Vee with Y; auto with zfc.
Qed.
Hint Resolve IN_Power_INC: zfc.

Lemma IN_Succ_INC : forall E X : Ens, IN X (Succ E) -> INC X E.
unfold Succ in |- *.