Open Scope nat_scope.
(* IDEM, LARGE *)

Theorem exds_orb_eq_ex_large :forall(m:fmap)(z t:dart),
inv_hmap m -> exd m z ->
let s:= Iter_orb m z in
let p:= Iter_upb m z in
(exds s t <-> exists i:nat, Iter (f m) i z = t).