code: def create_xml_string(self):
          
          root = self.create_xml()
  
          xml = '<?xml version="1.0" encoding="UTF-8"?>\n' + tostring(
              root, pretty_print=True
          )
          return xml
docs: Create a UNTL document in a string from a UNTL metadata
          root object.
  
          untl_xml_string = metadata_root_object.create_xml_string()