proposition: Lemma cubic_eq' : cubic -> forall x, node (node x) = face (edge x).
proof: Proof. by move=> cubicG x; rewrite -{1}[x]edgeK cubic_eq. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)