proposition: Lemma gtouch_edge d : gtouch (halfg d) (halfg (gedge d)).
proof: Proof. rewrite halfg_edge; case: (halfg d) => x y. by case: oddgP; rewrite /= !ler_add2l !ltr_add2l. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)