proposition: Lemma gchop_face_node d : gchop (gface (gnode d)) = gchop (gface (gface d)).
proof: Proof. rewrite /gchop !halfg_face !oddg_face halfg_node oddg_node. by case: (halfg d) (oddgP d) => x y [] //=; rewrite addr0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)