proposition: Lemma gchop_chop1 d : subpred (gchop d) (gchop1 d).
proof: Proof. rewrite /gchop1 /gchop !halfg_face halfg_edge !oddg_face oddg_edge ccw4. case: (halfg d) => x0 y0 [x y]. by case: oddgP => /= d_p; rewrite ltW // -?ltr_subr_addr ltz_addr1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)