def _metric(method, aggregator, sort):
    """Return 3-element list defining a metric, an aggregator and a sort order.

    .. note::

        This is a private function called by :func:`_add_default_metrics`.

    :param method:     image comparison metric (see :ref:`exquires-compare`)
    :param aggregator: data aggregator (see :ref:`exquires-aggregate`)
    :param sort:       best-to-worst order (`0`: ascending, `1`: descending)
    :type method:      `string`
    :type aggregator:  `string`
    :type sort:        `integer`

    :return:           metric, aggregator, and sort order
    :rtype:            `list`

    """
    return [' '.join(['exquires-compare', method, '{0} {1}']), ' '.join([
        'exquires-aggregate', aggregator, '{0}']), sort]
