proposition: Lemma cubic_neq : cubic -> forall x, (node x == x) = false.
proof: Proof. by move/cubicP=> cubicG x; apply/idPn; case: (cubicG x). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)