Parse a set-like header and return a
    :class:`~werkzeug.datastructures.HeaderSet` object:

    >>> hs = parse_set_header('token, "quoted value"')

    The return value is an object that treats the items case-insensitively
    and keeps the order of the items:

    >>> 'TOKEN' in hs
    True
    >>> hs.index('quoted value')
    1
    >>> hs
    HeaderSet(['token', 'quoted value'])

    To create a header from the :class:`HeaderSet` again, use the
    :func:`dump_header` function.

    :param value: a set header to be parsed.
    :param on_update: an optional callable that is called every time a
                      value on the :class:`~werkzeug.datastructures.HeaderSet`
                      object is changed.
    :return: a :class:`~werkzeug.datastructures.HeaderSet`

def parse_set_header(value, on_update=None):
    """Parse a set-like header and return a
    :class:`~werkzeug.datastructures.HeaderSet` object:

    >>> hs = parse_set_header('token, "quoted value"')

    The return value is an object that treats the items case-insensitively
    and keeps the order of the items:

    >>> 'TOKEN' in hs
    True
    >>> hs.index('quoted value')
    1
    >>> hs
    HeaderSet(['token', 'quoted value'])

    To create a header from the :class:`HeaderSet` again, use the
    :func:`dump_header` function.

    :param value: a set header to be parsed.
    :param on_update: an optional callable that is called every time a
                      value on the :class:`~werkzeug.datastructures.HeaderSet`
                      object is changed.
    :return: a :class:`~werkzeug.datastructures.HeaderSet`
    """
    if not value:
        return HeaderSet(None, on_update)
    return HeaderSet(parse_list_header(value), on_update)