def xy_to_z(xy):
    """
    Returns the *z* coordinate using given *xy* chromaticity coordinates.

    Parameters
    ----------
    xy : array_like
        *xy* chromaticity coordinates.

    Returns
    -------
    numeric
        *z* coordinate.

    References
    ----------
    .. [2]  `RP 177-1993 SMPTE RECOMMENDED PRACTICE -
            Television Color Equations: 3.3.2
            <http://car.france3.mars.free.fr/HD/INA-%2026%20jan%2006/SMPTE%20normes%20et%20confs/rp177.pdf>`_,  # noqa
            DOI: http://dx.doi.org/10.5594/S9781614821915

    Examples
    --------
    >>> xy_to_z((0.25, 0.25))
    0.5
    """

    return 1 - xy[0] - xy[1]