Proof
fun (F G X : Ens) (c : Comp_prop F G X) =>
Comp_inter_yz F G (PR1 X) (Comp_INTER F G X) (PR2 X)
(Comp_INTER_pr1 F G X c).

(* Specifications: *)
Theorem Comp_prop_th1 :
forall (F G X : Ens) (c : Comp_prop F G X), IN (PR1 X) (Domain F).