Definition R3
 (m:fmap)(z t:dart)(s:set):Prop:=
  ~ exds s t ->
   let sr := Iter_rem_aux m z s in
    ~ exds sr t.

(* Specifications: *)
Lemma LR3:forall(m:fmap)(z t:dart)(s:set), ~ exds s t -> let sr := Iter_rem_aux m z s in ~ exds sr t.