(* OK: *)

Lemma rem_2_steps :forall(m:fmap)(z:dart)(s:set),
inv_hmap m ->
let sr:= Iter_rem_aux m z s in
let nr:= Iter_upb_aux m z s in
(card sr + 2 <= card s ->
exds (Ds s (Iter (f m) (ndN m - card s) z))
(Iter (f m) (ndN m + 1 - card s) z))%nat.