proposition: Lemma edges_at_del (G : pre_graph) (z x : VT) : edges_at (G \ z) x = edges_at G x `\` edges_at G z.
proof: Proof. apply/fsetP => k. by rewrite !(edges_atE,inE) !incident_delv !andbA. Qed.
imports: HB(structures) Coq(Relation_Definitions Morphisms RelationClasses) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij equiv) GraphTheory(setoid_bigop structures pttdom rewriting) GraphTheory(finmap_plus)