proposition: Lemma Singleton_iff: forall U x y, Ensembles.In U (Singleton U x) y <-> x = y.
proof: Proof. intros; split; intro. + inversion H; auto. + subst; constructor. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext