public function setClassificationMapping($propertyUri, $classificationValue)
    {
        if (empty($propertyUri) || ! is_string($propertyUri)) {
            throw new MetadataExtractionException(__('PropertyUri to map has to a valid uri'));
        }
        if (empty($classificationValue) || ! is_string($classificationValue)) {
            throw new MetadataExtractionException(__('Classification value to map has to a string'));
        }
        $this->classification[$propertyUri] = $classificationValue;
    }