static class CombinedSet<A> extends AbstractSet<A> {
  new L<Set<A>> sets;

  *() {}
  *(Set<A> sets) { this.sets.addAll(sets); }

  public int size() { ret lengthLevel2(sets); }
  public Iterator<A> iterator() { fail(); }
  public bool contains(O o) {
    for (Set<A> set : sets)
      if (set != null && set.contains(o))
        true;
    false;
  }
}