public function getKeys()
    {
        /** @var \DOMNodeList $tmp */
        $transUnits = $this->getXPath()->query('/xliff:xliff/xliff:file/xliff:body/xliff:trans-unit');

        $result = array();

        if ($transUnits->length > 0) {
            /** @var \DOMElement $element */
            foreach ($transUnits as $element) {
                if (!$this->getAttribute($element, 'id')) {
                    throw new \Exception('Empty Id: ' . var_export($element, true));
                }
                $result[] = (string) $this->getAttribute($element, 'id');
            }
        }

        return $result;
    }