proposition: Lemma Same_set_sym: forall A (S1 S2 : Ensemble A), Same_set S1 S2 -> Same_set S2 S1.
proof: Proof. intros; destruct H; split; auto. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext