proposition: Lemma edges_at_remove_edges (G : pre_graph) z E : edges_at (remove_edges G E) z = edges_at G z `\` E.
proof: Proof. apply/fsetP => e. by rewrite !(inE,edges_atE) -andbA incident_dele. 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)