proposition: Lemma halfg_iter_face i d : halfg (iter i gface d) = halfg d.
proof: Proof. by elim: i => //= i IHi; rewrite halfg_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)