Append the extra substring to the node content. NOTE: In
          contrast to xmlNodeSetContentLen(), @content is supposed to
          be raw text, so unescaped XML special chars are allowed,
           entity references are not supported.

def addContentLen(self, content, len):
        """Append the extra substring to the node content. NOTE: In
          contrast to xmlNodeSetContentLen(), @content is supposed to
          be raw text, so unescaped XML special chars are allowed,
           entity references are not supported. """
        libxml2mod.xmlNodeAddContentLen(self._o, content, len)