def _merge_simple(adjustment_lists, front_idx, back_idx):
    """
    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.
    """
    if len(adjustment_lists) == 1:
        return list(adjustment_lists[0])
    else:
        return adjustment_lists[front_idx] + adjustment_lists[back_idx]
