proposition: Lemma gchop_shift d : gchop (gface (gedge (gface d))) = gchop d.
proof: Proof. by rewrite -gnode_face gchop_face_node gface4. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)