code: final DataSet theEmptySet() {
      if (theEmptyOne == null) {
        theEmptyOne = new DataSet(this, EMPTY_ROW_LIST);
        theEmptyOne.setReadOnly();
      }
      return theEmptyOne;
    }
docs: Return an empty, read-only data set,
  for use by {@link JDBDT#empty(DataSource)}
  @return Empty, read-only data set.