(* IF t IS REMOVED, THERE EXISTS i < nr, S.T. t IS THE ith ITERATED: OK *)

Lemma ex_i :forall(m:fmap)(z t:dart),
inv_hmap m -> exd m z -> exd m t ->
let sr:= Iter_rem m z in
let nr:= Iter_upb m z in
~ exds sr t ->
{i : nat | (i < nr)%nat /\ Iter (f m) i z = t}.