(********** Union ************)


Definition Union (F : Ens) : Ens := REPLACEMENT (b_in_Union_a F).

(* Specifications: *)
Theorem Union_pr1 : forall (F X : Ens) (h : EXISTS (fun U : Ens => nested_IN X U F)), IN X (Union F).
Theorem Union_pr2 : forall (F X : Ens) (i : IN X (Union F)), EXISTS (fun U : Ens => nested_IN X U F).