Qed.
Hint Resolve IN_Sing IN_Sing_EQ: zfc.

(* Specifications: *)
Theorem Sing_sound : forall A A' : Ens, EQ A A' -> EQ (Sing A) (Sing A').
unfold Sing in |- *; intros; apply EQ_tran with (Paire A A');
auto with zfc.
Theorem Sing_sound : forall A A' : Ens, EQ A A' -> EQ (Sing A) (Sing A').
unfold Sing in |- *; intros; apply EQ_tran with (Paire A A');
auto with zfc.