code: def write_xmlbif(self, filename):
          
          with open(filename, 'w') as fout:
              fout.write(self.__str__())
docs: Write the xml data into the file.
  
          Parameters
          ----------
          filename: Name of the file.
  
          Examples
          -------
          >>> writer = XMLBIFWriter(model)
          >>> writer.write_xmlbif(test_file)