apply EQ_tran with (Power (inj E')).
apply Power_sound; assumption.
apply EQ_sym; apply Power_sound_inj.
Qed.
(* Just for fun : a proof that there is no set of all sets, using       *)
(* Russell's paradox construction                                       *)
(* There, of course, are other proofs (foundation, etc)                 *)

Theorem Russell : forall E : Ens, (forall E' : Ens, IN E' E) -> False.
intros U HU.