protected function tokenizeDOM($node, &$tokens, $config)
    {
        $level = 0;
        $nodes = array($level => new HTMLPurifier_Queue(array($node)));
        $closingNodes = array();
        do {
            while (!$nodes[$level]->isEmpty()) {
                $node = $nodes[$level]->shift(); // FIFO
                $collect = $level > 0 ? true : false;
                $needEndingTag = $this->createStartNode($node, $tokens, $collect, $config);
                if ($needEndingTag) {
                    $closingNodes[$level][] = $node;
                }
                if ($node->childNodes && $node->childNodes->length) {
                    $level++;
                    $nodes[$level] = new HTMLPurifier_Queue();
                    foreach ($node->childNodes as $childNode) {
                        $nodes[$level]->push($childNode);
                    }
                }
            }
            $level--;
            if ($level && isset($closingNodes[$level])) {
                while ($node = array_pop($closingNodes[$level])) {
                    $this->createEndNode($node, $tokens);
                }
            }
        } while ($level > 0);
    }