proposition: Lemma halfg_edge p : halfg (gedge p) = halfg p - arcg (ccw (oddg p)).
proof: Proof. by rewrite -gnode_face halfg_node halfg_face oddg_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)