private static function init()
 {
     if (self::$NULL_SCOPE === NULL) {
         self::$NULL_SCOPE = new Scope();
         self::$DEFAULT_SCOPES = array();
         foreach (self::$DEFAULT_SCOPE_VALUES as $defaultUnitTag => $defaultValue) {
             if (strlen($defaultValue) > 0) {
                 self::$DEFAULT_SCOPES[$defaultUnitTag] = self::parseValue($defaultValue);
             } else {
                 self::$DEFAULT_SCOPES[$defaultUnitTag] = self::$NULL_SCOPE;
             }
         }
     }
 }