proposition: Lemma oddgP d : is_oddg (oddg d).
proof: Proof. by case: d => x y /=; rewrite !modz2; do 2!case: odd; constructor. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)