proposition: Lemma size_cpring_ecpU : size (cpring ecpU) = (size (cpring x0)).+2.
proof: Proof. by rewrite cpring_ecpU /= size_map. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)