persistable sclass LinkedTreeSet<A> extends AbstractSet<A> {
  Set<A> set;
  new LinkedList<A> list;
  
  *() { set = new TreeSet; }
  *(Set<A> *set) {}

  public int size() { ret list.size(); }
  public Iterator<A> iterator() { ret list.iterator(); }
  public bool contains(O o) { ret set.contains(o); }
  
  public bool add(A a) {
    if (!set.add(a)) false;
    ret true with list.add(a);
  }
  
  // TODO: optimize
  public bool remove(O o) {
    if (!set.remove(o)) false;
    ret true with list.remove(o);
  }
}