@Override
  public void writeExternal(Element element) {
    StringBuilder result = new StringBuilder();
    String sep = "";
    for (Float proportion : proportions) {
      result.append(sep);
      result.append(proportion);
      sep = ",";
    }
    element.setAttribute(ATTRIBUTE_PROPORTIONS, result.toString());
    element.setAttribute(ATTRIBUTE_VERSION, DATA_VERSION);
  }
@Override
  public void writeExternal(Element element)