simple induction E; intros A f r.
intros E' E'' i H.
elim (IN_EXType (sup A f) E'' i).
intros a e; simpl in a.
simpl in e.
cut (IN E' E''); auto with zfc.
intros i'.
cut (IN E' (f a)); auto with zfc.
intros i0.
elim (IN_EXType (f a) E' i0).
intros b e'.
simpl in |- *.
cut (forall x : A, IN (pi2 (f a) b) (f x)).
intros.
exists
(dep_i A
(fun a : A =>
depprod (pi1 (f a))
(fun b : pi1 (f a) => forall x : A, IN (pi2 (f a) b) (f x))) a
(dep_i (pi1 (f a))
(fun b : pi1 (f a) => forall x : A, IN (pi2 (f a) b) (f x)) b H0)).
simpl in |- *.
auto with zfc.
auto with zfc.
intros.
apply IN_sound_left with E'.
auto with zfc.
apply H.
auto with zfc.
simpl in |- *.
exists x; auto with zfc.
apply IN_sound_right with E''; auto with zfc.
Qed.
(*  The powerset and its axioms   *)
Definition Power (E : Ens) : Ens :=
match E with
| sup A f =>
sup _
(fun P : A -> Prop =>
sup _
(fun c : depprod A (fun a : A => P a) =>
match c with
| dep_i a p => f a
end))
end.

Theorem IN_Power_INC : forall E E' : Ens, IN E' (Power E) -> INC E' E.
simple induction E.