code: def copy_with_additional_key(self, key: Any) -> 'RandomnessStream':
          
          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)
docs: 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.