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