proposition: Lemma val_gmnode u : u \in gm_inner -> val (node u) = gedge (gnode u).
proof: Proof. by rewrite inE => bb_u; rewrite val_insubd gmgridE -halfg_face gnodeK bb_u. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph order ssralg ssrnum ssrint) fourcolor(hypermap geometry color coloring patch snip grid) fourcolor(matte)