Proof
fun (A F U : Ens) (i : IN U (restricted_to A F)) =>
Set_Of_th2 F (fun M : Ens => IN (PR1 M) A) U i.

(* Specifications: *)
Theorem restricted_to_th3 :
forall (A F U : Ens) (i : IN U (restricted_to A F)), IN U F.