proposition: Lemma edges_atC G e x y u : edges_at (G ∔ [e,x,u,y]) =1 edges_at (G ∔ [e,y,u°,x]).
proof: Proof. move => z. apply/fsetP => f. by rewrite !edges_atE /= incident_flip_edge. 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)