proposition: Lemma oddg_node d : oddg (gnode d) = ccw (oddg d).
proof: Proof. by rewrite -(oddg_add2 (arcg (oddg d))) addrCA addrK oddg_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)