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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 add_checkpoint_file(self,filename):
 """ Add filename as a checkpoint file for this DAG node @param filename: checkpoint filename to add """

    if filename not in self.__checkpoint_files:
        self.__checkpoint_files.append(filename)
        if not isinstance(self.job(), CondorDAGManJob):
            if self.job().get_universe() == 'grid':
                self.add_checkpoint_macro(filename)