proposition: Lemma ccw_odd d : ccw (oddg d) = oddg (ccw d).
proof: Proof. by case: d => x y /=; rewrite -modzDmr -modzNm modz2; case: (odd _). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)