<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def regular_folder() -> Callable: """ Returns a method that can be used in argument parsing to check the argument is a directory. :return: A method that can be used as a type in argparse. """


    def check_regular_directory(value_to_check):
        value_to_check = str(value_to_check)
        if not os.path.isdir(value_to_check):
            raise argparse.ArgumentTypeError("must be a directory.")
        return value_to_check

    return check_regular_directory