def argval(self):
        """ Returns the value of the arg (if any) or None.
        If the arg. is not an integer, an error be triggered.
        """
        if self.arg is None or any(x is None for x in self.arg):
            return None

        for x in self.arg:
            if not isinstance(x, int):
                raise InvalidArgError(self.arg)

        return self.arg