@Override
  public void readExternal(Element element) {
    proportions.clear();
    String prop = element.getAttributeValue(ATTRIBUTE_PROPORTIONS);
    String version = element.getAttributeValue(ATTRIBUTE_VERSION);
    if (prop != null && Objects.equals(version, DATA_VERSION)) {
      StringTokenizer tokenizer = new StringTokenizer(prop, ",");
      while (tokenizer.hasMoreTokens()) {
        String p = tokenizer.nextToken();
        proportions.add(Float.valueOf(p));
      }
    }
  }
@Override
  public void readExternal(Element element)