proposition: Lemma relabel_two a b: relabel (two_graph a b) ≃ two_graph (fv a) (fv b).
proof: Proof. etransitivity. apply relabel_union. apply union_iso; apply relabel_unit. Defined.
imports: HB(structures) Coq(Setoid Morphisms) mathcomp(all_ssreflect) GraphTheory(edone finite_quotient preliminaries bij equiv) GraphTheory(setoid_bigop structures pttdom mgraph ptt)