proposition: Lemma Union_Included {A: Type}: forall P Q R, Included (Union A P Q) R <-> Included P R /\ Included Q R.
proof: Proof. intros. unfold Included. pose proof Union_iff A 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