protected function getTrackingEventsDomElement()
    {
        // create container
        if ($this->trackingEventsDomElement) {
            return $this->trackingEventsDomElement;
        }
        
        $this->trackingEventsDomElement = $this->linearCreativeDomElement
            ->getElementsByTagName('TrackingEvents')
            ->item(0);

        if ($this->trackingEventsDomElement) {
            return $this->trackingEventsDomElement;
        }
        
        $this->trackingEventsDomElement = $this->linearCreativeDomElement
            ->ownerDocument
            ->createElement('TrackingEvents');

        $this->linearCreativeDomElement->firstChild->appendChild($this->trackingEventsDomElement);
        
        return $this->trackingEventsDomElement;
    }