'General transformation filter.
Equivalent to VideoClip.fl . The result is no more an
ImageClip, it has the class VideoClip (since it may be animated)'

def fl(self, fl, apply_to=None, keep_duration=True):

    if (apply_to is None):
        apply_to = []
    newclip = VideoClip.fl(self, fl, apply_to=apply_to, keep_duration=keep_duration)
    newclip.__class__ = VideoClip
    return newclip
