protected SortedSet<Integer> create(Integer[] elements) {
      return checkedCreate(nullCheckedTreeSet(elements));
    }
  }

  public static class ContiguousSetHeadsetGenerator extends AbstractContiguousSetGenerator {
    @Override
    protected SortedSet<Integer> create(Integer[] elements) {
      SortedSet<Integer> set = nullCheckedTreeSet(elements);
      int tooHigh = set.isEmpty() ? 0 : set.last() + 1;
      set.add(tooHigh);