(* IT SEEMS TO HAVE A PROBLEM WITH :  MA0.expo1_dec : forall (m : fmap) (z t : dart), inv_hmap m -> exd m z -> {MA0.expo1 m z t} + {~ MA0.expo1 m z t} *)
(* WE ALSO HAVE (EASILY): *)

Lemma succ_zi: forall(m:fmap)(z:dart)(i:nat),
inv_hmap m -> exd m z -> ~pred m zero z ->
(i + 1 < MA0.Iter_upb m z)%nat ->
let zi:= Iter (MA0.f m) i z in
succ m zero zi.