simple induction E; simpl in |- *.
intros A f r.
simple induction 1; simpl in |- *.
intros.
exists x; auto with zfc.
Qed.

(* Specifications: *)
(* The union axioms *)
Theorem IN_Union :
forall E E' E'' : Ens, IN E' E -> IN E'' E' -> IN E'' (Union E).
Theorem CIN_Union :
forall E E' E'' : Ens, CIN E' E -> CIN E'' E' -> CIN E'' (Union E).
Theorem CIN_Union :
forall E E' E'' : Ens, CIN E' E -> CIN E'' E' -> CIN E'' (Union E).