(* Now prove Contains_NN Naturals *)
(* prove the two inductive properties separately *)

(* Specifications: *)
Theorem Zero_in_Naturals : IN Zero Naturals.
Proof Naturals_pr1 Zero Zero_element_of_NN.
Theorem Nexts_in_Naturals :
forall (A : Ens) (h : IN A Naturals), IN (Next A) Naturals.