def _validate_case(case: str) -> None:
    """
    Validates the case argument for the `fmt_roman()` method.

    Args:
        case (str): The case argument to validate.

    Raises:
        ValueError: If the case argument is not 'upper' or 'lower'.
    """
    if case not in ["upper", "lower"]:
        raise ValueError(f"The `case` argument must be either 'upper' or 'lower' (not '{case}').")

    Validates the case argument for the `fmt_roman()` method.

    Args:
        case (str): The case argument to validate.

    Raises:
        ValueError: If the case argument is not 'upper' or 'lower'.
    
great_tables/_formats.py