proposition: Lemma gchop1_shift d : gchop1 (gface (gedge (gface d))) = gchop1 d.
proof: Proof. by rewrite -[gface d]gnode4 gnode_face /gchop1 !gnodeK gchop_face_node. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)