human
Write me a the Python program you requested given the following:
Complete the below
@audio_video_fx
def audio_fadein(clip, duration):

    ''''Return an audio (or video) clip that is first mute, then the
    sound arrives progressively over ``duration`` seconds.'
    '''
assistant
#Here is the Python program you requested.
    def fading(gf, t):
        gft = gf(t)
        if np.isscalar(t):
            factor = min(((1.0 * t) / duration), 1)
            factor = np.array([factor, factor])
        else:
            factor = np.minimum(((1.0 * t) / duration), 1)
            factor = np.vstack([factor, factor]).T
        return (factor * gft)
    return clip.fl(fading, keep_duration=True)