proposition: Lemma cardinal_eq_set : forall s1 s2, eq_set s1 s2 -> cardinal s1 = cardinal s2.
proof: Proof. intros s1 s2 s1_eq_s2; apply le_antisym; apply cardinal_subset; intros e e_in_si; generalize (s1_eq_s2 e); intuition. Qed.
imports: List more_list list_permut Arith