docs: def add_file_handler_to_root(log_fn):
      """Adds a file handler to the root logging.
  
      :param log_fn: the name of the log file.
  
      :type log_fn: str
  
      """
      file_handler = logging.FileHandler(log_fn, mode="w")
      file_handler.setFormatter(logging.Formatter(
          fmt="[%(asctime)s %(name)s %(levelname)s] %(message)s",
          datefmt="%Y-%m-%d %H:%M:%S",
      ))
      logging.root.addHandler(file_handler)