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(************ Powerset  **************)
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Definition Powerset (X : Ens) := REPLACEMENT (fun Y : Ens => SUB Y X).

Theorem Powerset_th3 : forall (A X Y : Ens) (x : IN X (Powerset A)), (* (y : (IN Y (Powerset A))) *) IN (intersection X Y) (Powerset A).