simpl in |- *; simple induction 1.
intros n e.
exists n; auto with zfc.
Qed.

(* Specifications: *)
Theorem IN_Nat_EXType :
forall (n : nat) (E : Ens),
IN E (Nat n) -> EXType _ (fun p : nat => EQ E (Nat p)).
Theorem IN_Nat_EXType :
forall (n : nat) (E : Ens),
IN E (Nat n) -> EXType _ (fun p : nat => EQ E (Nat p)).