code: def _last_bookmark(b0, b1):
      
      n = [None, None]
      _, _, n[0] = b0.rpartition(":")
      _, _, n[1] = b1.rpartition(":")
      for i in range(2):
          try:
              n[i] = int(n[i])
          except ValueError:
              raise ValueError("Invalid bookmark: {}".format(b0))
      return b0 if n[0] > n[1] else b1
docs: Return the latest of two bookmarks by looking for the maximum
      integer value following the last colon in the bookmark string.