(* IDEM: *)
(* CAUTION !! PROPAGATION OF exd m (F_1 m z) IN PLACE OF exd m (F m z) *)

(* Specifications: *)
Lemma F_1_F : forall (m:fmap)(z:dart),
inv_hmap m -> exd m z -> exd m (F m z) ->
F_1 m (F m z) = z.