proposition: Lemma rev_rev_ring r : rev_ring (rev_ring r) = r.
proof: Proof. by rewrite /rev_ring map_rev revK (mapK ee). 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)