code: def _read_next_timestep(self, ts=None):
      
  
      if self.ts.frame >= self.n_frames-1:
          raise IOError(errno.EIO, 'trying to go over trajectory limit')
      if ts is None:
          ts = self.ts
      ts.frame += 1
  
      self.zdock_inst._set_pose_num(ts.frame+1)
      ts._pos = self.zdock_inst.static_mobile_copy_uni.trajectory.ts._pos
  
      return ts
docs: copy next frame into timestep