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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 _set_defaults(self):
 """ Set some default values in the manifest. This method should be called after loading from disk, but before checking the integrity of the reference package. """

        self.contents.setdefault('log', [])
        self.contents.setdefault('rollback', None)
        self.contents.setdefault('rollforward', None)