Definition Countable_Union (A: Type) (P: nat -> Ensemble A) : Ensemble A :=
  fun x => exists i, P i x.Definition Non_Empty {U: Type} (A: Ensemble U): Prop := exists x, A x.Definition Binart_set_list (U: Type) (A B: Ensemble U): nat -> Ensemble U :=
  fun n => match n with | 0 => A | 1 => B | _ => Empty_set _ end.

Lemma Union_is_Countable_Union: forall {U: Type} (A B: Ensemble U), Same_set (Union _ A B) (Countable_Union _ (Binart_set_list _ A B)).