proposition: Lemma set_nil (T : finType) : [set x in Nil T] = set0.
proof: Proof. by apply/setP => x; rewrite inE. 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)