default values
    for optional function arguments, partly because of its
    less-confusing appearance in automatically generated
    documentation. Sentinels also function well as placeholders in queues
    and linked lists.

    .. note::

      By design, additional calls to ``make_sentinel`` with the same
      values will not produce equivalent objects.

      >>> make_sentinel('TEST') == make_sentinel('TEST')
      False
      >>> type(make_sentinel('TEST')) == type(make_sentinel('TEST'))
      False

    c