@Override
  public @Nullable Element getState() {
    if (myIgnoredUnknownFeatures.isEmpty()) return null;

    final Element ignored = new Element("ignored");
    for (UnknownFeature feature : myIgnoredUnknownFeatures) {
      final Element option = new Element("option");
      option.setAttribute(FEATURE_ID, feature.getFeatureType());
      option.setAttribute(IMPLEMENTATION_NAME, feature.getImplementationName());
      ignored.addContent(option);
    }
    return ignored;
  }
@Override
  public @Nullable Element getState()