Proof
fun (A F U : Ens) (i : IN U (restricted_to A F)) =>
Set_Of_th3 F (fun M : Ens => IN (PR1 M) A) U i.

Theorem restricted_to_th4 : forall A F : Ens, SUB (restricted_to A F) F.
Proof fun A F : Ens => restricted_to_th3 A F.