function __construct()
 {
     parent::__construct();
     $this->valid_parameters = array(RFC2445_XNAME => RFC2445_OPTIONAL);
     // The exception to the rule: this is not a static value, so we
     // generate it on-the-fly here. Guaranteed to be different for
     // each instance of this property, too. Nice.
     $this->val_default = Bennu::generate_guid();
 }