fact: Definition g' (dz : D') := if @boolP ((g dz) == y) is AltFalse p then Sub (g dz) p else (Sub x xDy) : G'.
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)