proposition: Lemma cface_icpY : {mono icpY : x y / cface x y}.
proof: Proof. by move=> x y; rewrite /icpY cface_icpN cface_icpU. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)