Proof
fun (A : Ens) (h : IN A Naturals) =>
Naturals_pr1 (Next A) (Nexts_element_of_NN A (Naturals_pr2 A h)).

(* Specifications: *)
Theorem Contains_NN_Naturals : Contains_NN Naturals.
Proof Build_Contains_NN Naturals Zero_in_Naturals Nexts_in_Naturals.