proposition: Lemma gchop_halfg d : gchop d (halfg d).
proof: Proof. by rewrite /gchop; case: (halfg d) (oddgP d) => x y [] /=. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)