(* USEFUL LEMMA, OK!: *)

Lemma betweenf1 : forall(m:fmap)(u v u' v':dart),
inv_hmap m -> exd m u' -> u <> u' -> v <> v' ->
(betweenf m u' u v' /\ betweenf m u' v v') ->
(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)).