proposition: Lemma gnodeK : cancel3 gnode gface gedge.
proof: Proof. by move=> p /=; rewrite -[p]gface4 gfaceK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)