human
I wrote the below code

    if (h is None):
        h = clip.h
    if (w is None):
        w = clip.w
    xmax = ((clip.w - w) - 1)
    ymax = ((clip.h - h) - 1)
    def f(gf, t):
        x = int(max(0, min(xmax, (x_start + round((x_speed * t))))))
        y = int(max(0, min(ymax, (y_start + round((y_speed * t))))))
        return gf(t)[y:(y + h), x:(x + w)]
    return clip.fl(f, apply_to=apply_to)

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

def scroll(clip, h=None, w=None, x_speed=0, y_speed=0, x_start=0, y_start=0, apply_to='mask'):