code: def xml(self):
          
  
          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
docs: the xml string representation.
  
          :return: the xml
          :rtype: str