proposition: Lemma Union_spec: forall A (v: A) P Q, Union _ P Q v <-> P v \/ Q v.
proof: Proof. intros. split; intros. + inversion H; 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