proposition: Lemma Union_assoc: forall A P Q R, Same_set (Union A (Union A P Q) R) (Union A P (Union A Q R)).
proof: Proof. intros. rewrite Same_set_spec; hnf; intros. rewrite !Union_spec. tauto. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext