code: def _apply_transformation(inputs):
      
      ts, transformation, extend_collection, clear_redo = inputs
      new = ts.append_transformation(transformation, extend_collection,
                                     clear_redo=clear_redo)
      o = [ts]
      if new:
          o.extend(new)
      return o
docs: Helper method for multiprocessing of apply_transformation. Must not be
      in the class so that it can be pickled.
  
      Args:
          inputs: Tuple containing the transformed structure, the transformation
              to be applied, a boolean indicating whether to extend the
              collection, and a boolean indicating whether to clear the redo
  
      Returns:
          List of output structures (the modified initial structure, plus
          any new structures created by a one-to-many transformation)