proposition: Lemma null_or_valid: forall (pg: PreGraph V E) {ma: MathGraph pg is_null} x, weak_valid pg x -> {is_null x} + {vvalid pg x}.
proof: Proof. intros. destruct (is_null_dec x); [left | right]; auto. unfold weak_valid in H. tauto. Qed.
imports: CertiGraph.lib.Coqlib CertiGraph.lib.Ensembles_ext CertiGraph.lib.EquivDec_ext CertiGraph.lib.List_ext CertiGraph.lib.relation_list CertiGraph.lib.Equivalence_ext CertiGraph.graph.graph_model CertiGraph.graph.graph_gen