proposition: Lemma vertex_at_not_null: forall (x: addr) (gx: addr * addr * addr), @derives pred _ (vertex_at x gx) (!! (null <> x)).
proof: Proof. intros. destruct (classic (null = x)). + subst x. apply derives_trans with FF. apply vertex_at_not_null. apply FF_left. + apply prop_right; auto. Qed.
imports: CertiGraph.lib.Ensembles_ext Coq.Lists.List VST.msl.seplog VST.msl.log_normalize VST.msl.ramification_lemmas VST.msl.Coqlib2 CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext CertiGraph.lib.relation_list CertiGraph.lib.Morphisms_ext CertiGraph.msl_ext.log_normalize CertiGraph.msl_ext.iter_sepcon CertiGraph.graph.graph_model CertiGraph.graph.path_lemmas CertiGraph.graph.reachable_computable CertiGraph.graph.reachable_ind CertiGraph.graph.graph_gen CertiGraph.graph.graph_relation CertiGraph.graph.subgraph2 CertiGraph.graph.dag CertiGraph.graph.weak_mark_lemmas CertiGraph.graph.graph_morphism CertiGraph.graph.local_graph_copy CertiGraph.msl_application.Graph CertiGraph.msl_application.GraphBin CertiGraph.msl_application.Graph_Copy Coq.Logic.Classical