(** From this, we can conclude: *)

Theorem powerset :
forall (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) (B : Rel Y) (b : Y),
iff (ELT Y B b (sum X (X -> Prop)) (POWER X A a) (out X (X -> Prop)))
(SUB Y B b X A a).