proposition: Lemma Included_trans: forall {A} (P Q R: Ensemble A), Included P Q -> Included Q R -> Included P R.
proof: Proof. unfold Included, Ensembles.In. intros; firstorder. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext