proposition: Lemma emb_invK : cancel emb_inv f.
proof: Proof. by move => z; rewrite f_iinv. Qed.
imports: mathcomp(all_ssreflect) fourcolor(hypermap geometry color coloring walkup combinatorial4ct cfmap) GraphTheory(edone preliminaries bij digraph sgraph) GraphTheory(connectivity minor hmap_ops smerge hcycle arc)