(* THEN, SYMMETRICALLY: OK, TERRIBLE !! *)

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)).