proposition: Lemma cubic_Euler : plain_cubic_connected -> planar G = (6 * fcard face G == #|G| + 12).
proof: Proof. by do 2![case] => *; rewrite (@quasicubic_Euler nil). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)