将查询字符串映射到Kohana 3.2中的控制器方法参数

我想设置我的路由，使url如word/startswith?prefix=ab映射到以下代码

class Controller_Word extends Controller_Mytemplate {
    public function action_startswith($fragment){
     /// Here $fragment should get "ab" value.
    }
}

还有谁能告诉我如何在Kohana 3.2控制器中获得get数据?对于上面的场景，我可以通过

class Controller_Word extends Controller_Mytemplate {
    public function action_startswith($fragment=null){
        if($fragment==null){
            /// set $fragment to $_GET['prefix'] but in Kohana way.
        }
    }
}