proposition: Lemma gnode_face d : gnode (gface d) = gedge d.
proof: Proof. by rewrite [RHS]addrA -oddg_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)