proposition: Lemma union_C G H: G ⊎ H ≃ H ⊎ G.
proof: Proof. exists bij_sumC bij_sumC xpred0. abstract by split; case. Defined.
imports: Coq(Morphisms RelationClasses) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij) GraphTheory(equiv digraph setoid_bigop structures)