(** From PAIR_eqv1 and PAIR_eqv2, we easily get that the pointed graphs (X, A, a) and (Y, B, b) are elements of the unordered pair. *)

(* Specifications: *)
Lemma pairing_intro1 :
forall (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) (B : Rel Y) (b : Y),
ELT X A a (sum X Y) (PAIR X A a Y B b) (out X Y).
Lemma pairing_intro2 :
forall (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) (B : Rel Y) (b : Y),
ELT Y B b (sum X Y) (PAIR X A a Y B b) (out X Y).