(*    (y : (IN Y (Powerset A)))  *)
Proof power1_Out2.
Section power2.
Variable A X : Ens.
Hypothesis i : IN X A.
Let step1 : SUB (Singleton X) A.
Proof Singleton_th1 X A i.

(* Specifications: *)
Lemma power2_Out : IN (Singleton X) (Powerset A).
Proof Powerset_pr1 A (Singleton X) step1.