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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def save_ext(self):
 """Write the internal data into an external data file."""

        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)