proposition: Lemma planar_WalkupE : planar G -> planar G'.
proof: Proof. by rewrite /planar -!leqn0 => /(leq_trans le_genus_WalkupE). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap)