Proof
fun (B : Ens) (i : IN B Naturals) =>
naturals_main_ind1 B (naturals_main_th1 B i).

(* Specifications: *)
Theorem naturals_main_th2_rewrite :
forall (B : Ens) (i : IN B Naturals) (A : Ens) (j : IN A B), StrictSUB A B.