code: def get_downbeat_steps(self):
          
          if self.downbeat is None:
              return []
          downbeat_steps = np.nonzero(self.downbeat)[0].tolist()
          return downbeat_steps
docs: Return the indices of time steps that contain downbeats.
  
          Returns
          -------
          downbeat_steps : list
              The indices of time steps that contain downbeats.