proposition: Lemma size_cpring_ecpN : size (cpring ecpN) = (size (cpring x0)).-2.+1.
proof: Proof. rewrite -(size_rot 1) rot1_cpring_ecpN drop_behead. by rewrite /= size_map !size_behead size_rot. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)