proposition: Lemma subgraph_sub : subgraph (subgraph_for con) G.
proof: Proof. exists val, val, xpred0. split => //=. Qed.
imports: Coq(Morphisms RelationClasses) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij) GraphTheory(equiv digraph setoid_bigop structures)