proposition: Lemma adjacent_symm: forall g u v, adjacent g u v -> adjacent g v u.
proof: Proof. intros. destruct H. exists x. destruct H. destruct H0. split. apply H. right. apply H0. split. apply H. left. apply H0. 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