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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 get_checkpoint_files(self):
 """ Return a list of checkpoint files for this DAG node and its job. """

    checkpoint_files = list(self.__checkpoint_files)
    if isinstance(self.job(), CondorDAGJob):
        checkpoint_files = checkpoint_files + self.job().get_checkpoint_files()
    return checkpoint_files