Section comp_prop_3.
Variable F G X : Ens.
Hypothesis c : Comp_prop F G X.
Let step1 : is_a_pair X.
Proof Comp_pair F G X c.
Let step2 : IN (PR1 X) (Domain F).
Proof Comp_prop_th1 F G X c.
Let step3 : IN (PR2 X) (Range G).
Proof Comp_prop_th2 F G X c.
Let step4 : in_cartesian (Domain F) (Range G) X.
Proof Build_in_cartesian (Domain F) (Range G) X step1 step2 step3.
Proof Cartesian_pr1 (Domain F) (Range G) X step4.
End comp_prop_3.

Theorem Comp_prop_th3 :
forall (F G X : Ens) (c : Comp_prop F G X),
IN X (Cartesian (Domain F) (Range G)).