proposition: Lemma gmedge2 : involutive gmedge.
proof: Proof. by move=> u; apply/val_inj; rewrite /= gedge2. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph order ssralg ssrnum ssrint) fourcolor(hypermap geometry color coloring patch snip grid) fourcolor(matte)