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

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"
        })