proposition: Lemma ext2_mring_simple : uniq (map end0g ext2_mring).
proof: Proof. rewrite -(rot_uniq 1) /ext2_mring; case: ext2_P => -[i r] Dr; rewrite /= in Dr. have:= mring_simple m; rewrite -(rot_uniq i) -(rot_uniq 1). have:= mring_equad m'f4d; rewrite -(mem_rot i) -(mem_rot 1) -!map_rot {i}Dr. rewrite !rot1_cons !map_rcons end0g_edge -end0g_face /= inE => /norP[_ ->]. by case/andP. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path order ssralg ssrnum ssrint div intdiv) fourcolor(grid)