proposition: Lemma gface4 d : gface (gface (gface (gface d))) = d.
proof: Proof. by rewrite gface_def !halfg_face !oddg_face ccw4 halfgK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)