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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 unique_size(self):
 """ Size of ONLY this particular layer :return: int or None """

        self._virtual_size = self._virtual_size or \
                             graceful_chain_get(self.data, "VirtualSize", default=0)
        try:
            return self._virtual_size - self._shared_size
        except TypeError:
            return 0