private function printActiveFormattingElements() {
        if (!$this->a_formatting) {
            return;
        }
        $names = array();
        foreach ($this->a_formatting as $node) {
            if ($node === self::MARKER) {
                $names[] = 'MARKER';
            } else {
                $names[] = $node->tagName;
            }
        }
        echo "  -> active formatting [" . implode(', ', $names) . "]\n";
    }