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