proposition: Lemma edges_at_oarc e x y z u : e \in edges_at G z -> oarc G e x u y -> x = z \/ y = z.
proof: Proof. move => Iz [Ee [b] [<- <- _]]. case/edges_atP : Iz => _ [[|]]; case: b; tauto. 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)