End gd1a.
Section gd1b.
Hypothesis p2 : IN (PAIR X Y) G.
Let step3 : IN X (Domain G).
Proof Domain_th1 G X Y p2.

(* Specifications: *)
Lemma gd1b_Out : IN X (union (Domain F) (Domain G)).
Proof union_th2 (Domain F) (Domain G) X step3.