code: def _next_rotation_id(rotated_files):
      
      if not rotated_files:
          return 0
      else:
          highest_rotated_file = max(rotated_files, key=lambda x: x[1])
          return highest_rotated_file[1] + 1
docs: Given the hanoi_rotator generated files in the output directory,
      returns the rotation_id that will be given to the current file. If there
      are no existing rotated files, return 0.