proposition: Lemma map_embd_ring : map embd embd_ring = erc.
proof: Proof. apply: map_preim => _ /mapP[x rc_x ->]; rewrite codom_embd. by have:= edge_perimeter x; rewrite inE /= rc_x. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color chromogram coloring patch) fourcolor(snip revsnip kempe birkhoff contract)