proposition: Lemma halfg_node d : halfg (gnode d) = halfg d - arcg (oddg d).
proof: Proof. by rewrite -(halfg_face d) addrC -halfg_add2 addrCA subrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)