code: def get_empty_tracks(self):
          
          empty_track_indices = [idx for idx, track in enumerate(self.tracks)
                                 if not np.any(track.pianoroll)]
          return empty_track_indices
docs: Return the indices of tracks with empty pianorolls.
  
          Returns
          -------
          empty_track_indices : list
              The indices of tracks with empty pianorolls.