/**
  * get singleton instance
  *
  * @access public
  * @param int $a_usr_id user id
  * @return ilCalendarCategories
  * @static
  */
 public static function _getInstance($a_usr_id = 0)
 {
     if (self::$instance) {
         return self::$instance;
     }
     return self::$instance = new ilCalendarCategories($a_usr_id);
 }