proposition: Lemma oarc_edge_atL e x y u : oarc G e x u y -> e \in edges_at G x.
proof: Proof. case => E [b] [A B C]. rewrite !inE E /= /incident. existsb b. exact/eqP. 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)