Proof
fun (A B U : Ens) (i : IN U (Cartesian A B)) =>
in_cartesian_pair A B U (Cartesian_pr2 A B U i).

(* Specifications: *)
Theorem Cartesian_proj1_th :
forall (A B U : Ens) (i : IN U (Cartesian A B)), IN (PR1 U) A.