proposition: Lemma map_snipr_ring : map snipr snipr_ring = rev r.
proof: Proof. by apply: map_preim => x rx; rewrite codom_snipr !inE -mem_rev rx. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan color geometry patch)