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