code: def _merge_simple(adjustment_lists, front_idx, back_idx):
      
      if len(adjustment_lists) == 1:
          return list(adjustment_lists[0])
      else:
          return adjustment_lists[front_idx] + adjustment_lists[back_idx]
docs: Merge lists of new and existing adjustments for a given index by appending
      or prepending new adjustments to existing adjustments.
  
      Notes
      -----
      This method is meant to be used with ``toolz.merge_with`` to merge
      adjustment mappings. In case of a collision ``adjustment_lists`` contains
      two lists, existing adjustments at index 0 and new adjustments at index 1.
      When there are no collisions, ``adjustment_lists`` contains a single list.
  
      Parameters
      ----------
      adjustment_lists : list[list[Adjustment]]
          List(s) of new and/or existing adjustments for a given index.
      front_idx : int
          Index of list in ``adjustment_lists`` that should be used as baseline
          in case of a collision.
      back_idx : int
          Index of list in ``adjustment_lists`` that should extend baseline list
          in case of a collision.
  
      Returns
      -------
      adjustments : list[Adjustment]
          List of merged adjustments for a given index.