public function addClass($class)
    {
        $current = preg_split('%\s+%', $this->getAttribute('class'), 0, PREG_SPLIT_NO_EMPTY);
        $added = preg_split('%\s+%', $class, 0, PREG_SPLIT_NO_EMPTY);
        $current = array_merge($current, $added);
        $classes = implode(' ', $current);

        $this->setAttribute('class', $classes);
    }