proposition: Lemma cubic_ecpY : quasicubic (rev (cpring ecpY)).
proof: Proof. by apply: cubic_ecpN => //; apply: cubic_ecpU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)