@Override
  public IdBitSet clone() {
    try {
      IdBitSet clone = (IdBitSet)super.clone();
      if (bitSlots.length != maxNonZeroSlotIndex + 1) { // trim to size
        bitSlots = Arrays.copyOf(bitSlots, maxNonZeroSlotIndex + 1);
      }
      clone.bitSlots = bitSlots.clone();
      return clone;
    }
    catch (CloneNotSupportedException ex) {
      throw new AssertionError("Must not happen, since class implements Cloneable", ex);
    }
  }
@Override
  public IdBitSet clone()