proposition: Lemma proper_rev_ring r : proper_ring (rev_ring r) = proper_ring r.
proof: Proof. case lt2r: (2 < size r); first by rewrite !size_proper_ring ?size_rev ?size_map. by case: r lt2r => [|x [|y []]] //= _; rewrite ee eq_sym. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry patch sew snip color chromogram) fourcolor(coloring kempe)