private function setSimpleOperator($operator, $value)
    {
        if (empty($operator)) {
            throw new InvalidArgumentException('The operator must not be null' .
                ' or empty.');
        }

        if (is_null($value) || $value === '') {
            throw new InvalidArgumentException('The value string must not be' .
                ' null or empty');
        }

        $this->simpleOperator = $operator;
        $this->simpleValue = $value;
    }