proposition: Lemma Disjoint_comm: forall A P Q, Disjoint A P Q <-> Disjoint A Q P.
proof: Proof. intros. rewrite !Disjoint_spec. firstorder. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext