proposition: Lemma edge_in_set e (A : {set G}) x y : x \in A -> y \in A -> e \in edges x y -> e \in edge_set A.
proof: Proof. move => Hx Hy. rewrite !inE => /andP[/eqP->/eqP->]. by rewrite Hx. Qed.
imports: Coq(Morphisms RelationClasses) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij) GraphTheory(equiv digraph setoid_bigop structures)