Checks if the string `s` is a valid fraction-digits argument.

    Return True or False.

def chk_fraction_digits_arg(s):
    """Checks if the string `s` is a valid fraction-digits argument.

    Return True or False."""
    try:
        v = int(s)
        if v >= 1 and v <= 18:
            return True
        else:
            return False
    except ValueError:
        return False