proposition: Lemma arity_cface x y : cface x y -> arity x = arity y.
proof: Proof. by move=> xFy; apply: (eq_card (same_cface xFy)). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)