docs: def add_file_handler(logger, logFilename=None):
      """Adds file handler to logger.
  
      File is opened in "a" mode (append)
      """
      assert isinstance(logger, logging.Logger)
      ch = logging.FileHandler(logFilename, "a")
      # ch.setFormatter(logging._defaultFormatter) # todo may change to have same formatter as last handler of logger
      ch.setFormatter(_fmtr)
      logger.addHandler(ch)