@Override
  protected String checkValidity() {
    final XmlTag tag = (XmlTag)getXmlElement();
    if (tag != null && !tag.isValid()) {
      return "invalid root tag";
    }

    final String s = myParent.checkValidity();
    if (s != null) {
      return "root: " + s;
    }

    return null;
  }
@Override
  protected String checkValidity()