public function addClass($className) {
        $oldClassName    = $this->getAttributeValue('class');
        $oldClassNameArr = $oldClassName ? explode(' ', $oldClassName) : array();
        $classNameArr    = array_merge($oldClassNameArr, explode(' ', $className));
        // housekeeping
        $classNameArr    = array_unique(array_filter(array_map('trim', $classNameArr)));
        $this->setAttribute('class', implode(' ', $classNameArr));
    }