/**
  * SystemEventManager is singleton
  * 
  * @return SystemEventManager
  */
 public static function instance()
 {
     if (!isset(self::$_instance)) {
         $c = __CLASS__;
         self::$_instance = new $c();
     }
     return self::$_instance;
 }
 public function clearInstance()
 {
     self::$_instance = null;
 }