code: def save_ext(self):
          
          try:
              sequencemanager = hydpy.pub.sequencemanager
          except AttributeError:
              raise RuntimeError(
                  'The time series of sequence %s cannot be saved.  Firstly,'
                  'you have to prepare `pub.sequencemanager` correctly.'
                  % objecttools.devicephrase(self))
          sequencemanager.save_file(self)
docs: Write the internal data into an external data file.