'Time-transformation filter.
Applies a transformation to the clip\'s timeline
(see Clip.fl_time).
This method does nothing for ImageClips (but it may affect their
masks or their audios). The result is still an ImageClip.'

@outplace
def fl_time(self, time_func, apply_to=None, keep_duration=False):

    if (apply_to is None):
        apply_to = ['mask', 'audio']
    for attr in apply_to:
        if hasattr(self, attr):
            a = getattr(self, attr)
            if (a is not None):
                new_a = a.fl_time(time_func)
                setattr(self, attr, new_a)
