proposition: Lemma planar_ecpY : planar ecpY.
proof: Proof. apply: planar_ecpN => //. - exact: planar_ecpU. - exact: plain_ecpU. - exact: cubic_ecpU. exact: connected_ecpU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)