proposition: Lemma oddg_face d : oddg (gface d) = ccw (oddg d).
proof: Proof. by rewrite oddg_add subrK ccw_odd oddg_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)