proposition: Lemma Prop_join_Empty: forall {U} (A: Ensemble U), Prop_join A (Empty_set _) A.
proof: Proof. intros. split; intros. + rewrite Empty_set_spec. tauto. + rewrite Empty_set_spec in H0; auto. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext