proposition: Lemma halfg_face d : halfg (gface d) = halfg d.
proof: Proof. by rewrite halfg_add subrK ccw_odd (halfg_odd (oddgP _)) addr0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)