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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 uniquekeys(self, metakey=None):
 """ Get keys for a given metakey """

        if metakey is None:
            return self._uniquekeys
        else:
            if metakey not in self.metakeys():
                raise ValueError('metakey %s does not exist' % metakey)
            return self._uniquekeys[metakey]