code: def transpose(self, semitone):
          
          for track in self.tracks:
              if not track.is_drum:
                  track.transpose(semitone)
docs: Transpose the pianorolls of all tracks by a number of semitones, where
          positive values are for higher key, while negative values are for lower
          key. The drum tracks are ignored.
  
          Parameters
          ----------
          semitone : int
              The number of semitones to transpose the pianorolls.