proposition: Lemma gface_def d : gface d = halfg d *+ 2 + ccw (oddg d).
proof: Proof. by rewrite -halfg_face -oddg_face halfgK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)