proposition: Lemma Union_left_Disjoint: forall A P Q R, Disjoint A (Union A P Q) R <-> Disjoint A P R /\ Disjoint A Q R.
proof: Proof. intros. rewrite !Disjoint_spec. pose proof (fun x => Union_spec A x P Q). firstorder. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext