def set_value(self, pymux, value):
        """
        Take a string, and return an integer. Raise SetOptionError when the
        given text does not parse to a positive integer.
        """
        try:
            value = int(value)
            if value < 0:
                raise ValueError
        except ValueError:
            raise SetOptionError('Expecting an integer.')
        else:
            setattr(pymux, self.attribute_name, value)
Take a string, and return an integer. Raise SetOptionError when the
        given text does not parse to a positive integer.