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.

(* Specifications: *)
Lemma Union_is_Countable_Union: forall {U: Type} (A B: Ensemble U),
Same_set (Union _ A B) (Countable_Union _ (Binart_set_list _ A B)).
Lemma Intersection_is_Complement_Union (classic: forall P: Prop, P \/ ~ P): forall {U: Type} (A B: Ensemble U),
Same_set (Intersection _ A B) (Complement _ (Union _ (Complement _ A) (Complement _ B))).