docs: def get_walk_after(filename, optimize_after=None):
      """
      Figure out the which mtime to check against.
  
      If we have to look up the path return that.
      """
      if Settings.optimize_after is not None:
          return Settings.optimize_after
  
      dirname = os.path.dirname(filename)
      if optimize_after is None:
          optimize_after = _get_parent_timestamp(dirname, optimize_after)
      return _max_timestamps(dirname, True, optimize_after)