proposition: Lemma Union_iff: forall U A B x, Ensembles.In U (Union U A B) x <-> Ensembles.In U A x \/ Ensembles.In U B x.
proof: Proof. intros; split; intros. + apply Constructive_sets.Union_inv; auto. + destruct H; [apply Union_introl | apply Union_intror]; auto. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext