Section cpb1.
Variable U : Ens.
Hypothesis i : in_cartesian A B U.
Let step1 : is_a_pair U.
Proof in_cartesian_pair A B U i.
Let X := PR1 U.
Let Y := PR2 U.
Let step4 : EQ U (PAIR X Y).
Proof PAIR_proj_th1 U step1.
Let step5 : IN X A.
Proof in_cartesian_pr1 A B U i.
Let step6 : IN Y B.
Proof in_cartesian_pr2 A B U i.
Let step7 : IN (PAIR X Y) (PowerTotal (union A B)).
Proof PowerTotal_th2 A B X Y step5 step6.
Proof Extensionality U (PAIR X Y) (PowerTotal (union A B)) step4 step7.
End cpb1.
Let step8 : Bounded_By (PowerTotal (union A B)) (in_cartesian A B).
Proof cpb1_Out.
Proof Bounded_th2 (in_cartesian A B) (PowerTotal (union A B)) step8.
End in_cartesian_bounded_proof.
Proof in_cartesian_bounded_Out.
Definition Cartesian (A B : Ens) := REPLACEMENT (in_cartesian A B).

Theorem Cartesian_pr1 :
forall (A B M : Ens) (h : in_cartesian A B M), IN M (Cartesian A B).