(* EXTRACTION, OK: *)

Definition P4(m:fmap)(z t:dart)(s:set):Set:=
 inv_hmap m -> exd m z -> exds s t ->
   (card s <= ndN m)%nat ->
   let sr:= Iter_rem_aux m z s in
   let nr:= Iter_upb_aux m z s in
   ~ exds sr t ->
      {i:nat | (i < nr)%nat /\ Iter (f m) i z = t}.

Lemma Iter_f_1_Si:forall(m:fmap)(i:nat)(z:dart), inv_hmap m -> exd m z -> Iter (f_1 m) (S i) z = Iter (f_1 m) i (f_1 m z).