protected function processPrefix(\DOMElement $element, ContextInterface $context)
    {
        if ($element->hasAttribute('prefix') && ($context instanceof RdfaLiteContext)) {
            $prefixes = preg_split('/\s+/', $element->getAttribute('prefix'));
            while (count($prefixes)) {
                $prefix = rtrim(array_shift($prefixes), ':');
                $uri = array_shift($prefixes);
                $context = $context->registerVocabulary($prefix, $uri);
            }
        }

        return $context;
    }