proposition: Lemma out_edges_step: forall (pg: PreGraph Vertex Edge) x e, out_edges pg x e -> step pg x (dst pg e).
proof: Proof. unfold out_edges. intros. rewrite step_spec. firstorder. Qed.
imports: Coq.Logic.ProofIrrelevance CertiGraph.lib.Ensembles_ext CertiGraph.lib.EquivDec_ext CertiGraph.lib.List_ext CertiGraph.lib.relation_list CertiGraph.lib.Equivalence_ext