proposition: Lemma adjacent_err_hd: forall g u opt_v, adjacent_err g (Some u) opt_v -> adjacent_hd g u opt_v.
proof: Proof. destruct opt_v; intros; apply H. Qed.
imports: Coq.Logic.Classical CertiGraph.lib.List_ext CertiGraph.lib.EquivDec_ext CertiGraph.graph.graph_model CertiGraph.graph.graph_relation CertiGraph.graph.path_lemmas CertiGraph.graph.graph_gen CertiGraph.graph.FiniteGraph