@add_mask_if_none
def crossfadein(clip, duration):
 newclip = clip.copy()
 newclip.mask = clip.mask.fx(fadein, duration)
 return newclip

@ add_mask_if_none def crossfadein ( clip , duration ) : newclip = clip . copy ( ) newclip . mask = clip . mask . fx ( fadein , duration ) return newclip
makes the clip appear progressively .