//used to save persistent object to disk
	function getPersistentFieldPropertyDescriptors() {
		$result = array(); // anwsering reference to unset var may crash php
		$props =& $this->refPropertyDescriptors();
		if (empty($props))
			return $result;
		reset($props);
		foreach ($props as $name => $prop) {
			if ($prop->isFieldProperty() && $prop->getPersistent())
				$result[$name] = $prop;
		}

		return( $result );
	}