def xml(self):
        """
        the xml string representation.

        :return: the xml
        :rtype: str
        """

        xml = self._get_xml(False)
        for key in self.impact_functions_fields:
            value = str(self.data(key))
            element = Element(key)
            element.text = value

            xml += tostring(element, 'unicode')

        xml += '</provenance_step>'
        return xml
the xml string representation.

        :return: the xml
        :rtype: str