Proof
fun (F G X : Ens) (i : IN X (COMP F G)) =>
Comp_prop_th3 F G X (COMP_pr2 F G X i).

Theorem COMP_th3 :
forall (F G X : Ens) (i : IN X (COMP F G)),
IN (PAIR (PR1 X) (Comp_INTER F G X)) F.