human
I wrote the below code

    if (RGB is None):
        RGB = [1, 1, 1]
    if (RGB == 'CRT_phosphor'):
        RGB = [0.2125, 0.7154, 0.0721]
    (R, G, B) = ((1.0 * np.array(RGB)) / (sum(RGB) if preserve_luminosity else 1))
    def fl(im):
        im = (((R * im[:, :, 0]) + (G * im[:, :, 1])) + (B * im[:, :, 2]))
        return np.dstack((3 * [im])).astype('uint8')
    return clip.fl_image(fl)

What's a good function header?
assistant
#Here's a good function header.

def blackwhite(clip, RGB=None, preserve_luminosity=True):