proposition: Lemma cubic_precubic : cubic -> precubic.
proof: Proof. by move/subsetP=> cubicG; apply/subsetP => x /cubicG/eqnP <-; apply: leqnn. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)