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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 copy_with_additional_key(self, key: Any) -> 'RandomnessStream': """Creates a copy of this stream that combines this streams key with a new one. Parameters key : The additional key to describe the new stream with. Returns ------- RandomnessStream A new RandomnessStream with a combined key. """

        if self._for_initialization:
            raise RandomnessError('Initialization streams cannot be copied.')
        elif self._manager:
            return self._manager.get_randomness_stream('_'.join([self.key, key]))
        else:
            return RandomnessStream(self.key, self.clock, self.seed, self.index_map)