fact: Hypothesis simpleD : forall x y : D, x != y -> cnode x y -> ~~ cnode (edge x) (edge y).
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)