human
Complete the below
def fadein(clip, duration, initial_color=None):

    ''''Makes the clip progressively appear from some color (black by default),
    over ``duration`` seconds at the beginning of the clip. Can be used for
    masks too, where the initial color must be a number between 0 and 1.
    For cross-fading (progressive appearance or disappearance of a clip
    over another clip, see ``composition.crossfade``'
    '''

Give me a the Python program you requested?
assistant
#Here is the Python program you requested.
    if (initial_color is None):
        initial_color = (0 if clip.ismask else [0, 0, 0])
    initial_color = np.array(initial_color)
    def fl(gf, t):
        if (t >= duration):
            return gf(t)
        else:
            fading = ((1.0 * t) / duration)
            return ((fading * gf(t)) + ((1 - fading) * initial_color))
    return clip.fl(fl)