(* 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 ex_i_aux :forall(m:fmap)(z t:dart)(s: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}.