Section power2.
Variable A X : Ens.
Hypothesis i : IN X A.
Let step1 : SUB (Singleton X) A.
Proof Singleton_th1 X A i.
Proof Powerset_pr1 A (Singleton X) step1.
End power2.

Theorem Powerset_th4 :
forall (A X : Ens) (x : IN X A), IN (Singleton X) (Powerset A).