proposition: Lemma size_cpring_ecpY : size (cpring ecpY) = (size (cpring x0)).+1.
proof: Proof. by rewrite size_cpring_ecpN size_cpring_ecpU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)