proposition: Lemma size_proper_ring r : 2 < size r -> proper_ring r.
proof: Proof. by case: r => [|x [|y [|z r]]]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry patch sew snip color chromogram) fourcolor(coloring kempe)