Create Submenu For Yii Bootstrap

hi all . how to create submenu for below code:


$this->widget('bootstrap.widgets.TbButtonGroup', array(

'type' => 'primary', // '', 'primary', 'info', 'success', 'warning', 'danger' or 'inverse'

   	'buttons' => array(

                 	array('label' => 'Action', 'url' => '#'), // this makes it split <img src='http://www.yiiframework.com/forum/public/style_emoticons/default/smile.gif' class='bbc_emoticon' alt=':)' />

                 	array('items' => array(

                 	array('label' => 'Action', 'url' => '#'),

                 	array('label' => 'Another action', 'url' => '#'),

                 	array('label' => 'Something else', 'url' => '#'),

                 	array('label' => 'Separate link', 'url' => '#'),

      	)),

  	),

));