private function setListOperator($listOperator, array $values)
    {
        if (empty($listOperator)) {
            throw new InvalidArgumentException('The list operator must not' .
                ' be null or empty.');
        }

        if (empty($values)) {
            throw new InvalidArgumentException('The list of values must not' .
                ' be null or empty.');
        }

        $this->listOperator = $listOperator;
        $this->listValues = $values;
    }