proposition: Lemma pick_in_ring : {x | x \in r}.
proof: Proof. by case: r nt_r => // x p; exists x; rewrite mem_head. 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)