intros x; elim x; intros a aa; elim (e1 a); intros a' ea.
elim (EQ_EXType (f a) (f' a') ea aa); intros aa' eaa.
exists (dep_i A' (fun x : A' => pi1 (f' x)) a' aa'); simpl in |- *;
auto with zfc.
intros c'; elim c'; intros a' aa'; elim (e2 a'); intros a ea.
cut (EQ (f' a') (f a)).
2: auto with zfc.
intros ea'; elim (EQ_EXType (f' a') (f a) ea' aa'); intros aa eaa.
exists (dep_i A (fun x : A => pi1 (f x)) a aa); auto with zfc.
Qed.
(* The union construction is monotone w.r.t. inclusion   *)

Theorem Union_mon : forall E E' : Ens, INC E E' -> INC (Union E) (Union E').
unfold INC in |- *; intros E E' IEE E'' IEE''.