proposition: Lemma union_add_vlabel_l F G x a: F [tst x <- a] ⊎ G ≃ (F ⊎ G) [tst unl x <- a].
proof: Proof. Iso bij_id bij_id 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)