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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 berksfile(self):
 """Return this cookbook's Berksfile instance."""

        self.berks_path = os.path.join(self.path, 'Berksfile')
        if not self._berksfile:
            if not os.path.isfile(self.berks_path):
                raise ValueError("No Berksfile found at %s"
                                 % self.berks_path)
            self._berksfile = Berksfile(open(self.berks_path, 'r+'))
        return self._berksfile