Lemma betweenf2:forall(m:fmap)(u v u' v':dart), inv_hmap m -> exd m v' -> u <> u' -> v <> v' -> (betweenf m (cF m v') u (cF_1 m u') /\ betweenf m (cF m v') v (cF_1 m u')) -> (betweenf m u u' v /\ betweenf m u v' v \/ betweenf m (cF m v) u' (cF_1 m u) /\ betweenf m (cF m v) v' (cF_1 m u)).