Proof
fun (A F U : Ens) (i : IN U F) (j : IN (PR1 U) A) =>
Set_Of_th1 F (fun M : Ens => IN (PR1 M) A) U i j.

Theorem restricted_to_th2 :
forall (A F U : Ens) (i : IN U (restricted_to A F)), IN (PR1 U) A.