public function onUnaryExpression($expressionType, $child) {
        switch ($expressionType) {
            case ExpressionType::NEGATE:
                return $this->_prepareUnaryExpression(self::NEGATE, $child);
                break;
            case ExpressionType::NOT_LOGICAL:
                return $this->_prepareUnaryExpression(self::LOGICAL_NOT, $child);
                break;
            default:
                throw new \InvalidArgumentException('onUnaryExpression');
        }
    }