docs: def open_logfile(filename, mode='a'):
      """Open the named log file in append mode.
  
      If the file already exists, a separator will also be printed to
      the file to separate past activity from current activity.
      """
      filename = os.path.expanduser(filename)
      filename = os.path.abspath(filename)
      dirname = os.path.dirname(filename)
      if not os.path.exists(dirname):
          os.makedirs(dirname)
      exists = os.path.exists(filename)
  
      log_fp = open(filename, mode)
      if exists:
          log_fp.write('%s\n' % ('-'*60))
          log_fp.write('%s run on %s\n' % (sys.argv[0], time.strftime('%c')))
      return log_fp