fact: Definition gchop d : gregion := fun p => let: (Gpoint x0 y0, Gpoint x y) := (halfg d, p) in oddg_cases (oddg d) (y0 <= y) (x0 <= x) (x <= x0) (y <= y0).
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)