docs: def getValidReff(self, urn, inventory=None, level=None):
          """ Retrieve valid urn-references for a text
  
          :param urn: URN identifying the text
          :type urn: text
          :param inventory: Name of the inventory
          :type inventory: text
          :param level: Depth of references expected
          :type level: int
          :return: XML Response from the API as string
          :rtype: str
          """
          return self.call({
              "inv": inventory,
              "urn": urn,
              "level": level,
              "request": "GetValidReff"
          })