proposition: Lemma end0g_face d : end0g (gface d) = end1g d.
proof: Proof. by rewrite /end0g oddg_face halfg_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)