code: public static DitaClass getInstance(final Element elem) {
          if (elem == null) {
              return null;
          }
          final Attr attr = elem.getAttributeNode(ATTRIBUTE_NAME_CLASS);
          if (attr == null) {
              return null;
          }
          return getInstance(attr.getNodeValue());
      }
docs: Get class instance.
  @param elem element, may be {@code null}
  @return DITA class, {@code null} if the input didn't contains a class