/** @param string $aString must be alpanumeric */
	function addDerivedProp($name, $type, $readOnly=true, $minValue=null, $maxValue=null, $minLength=0, $maxLength=null, $classDir=null)
	{
		if (strlen($name)==0) {
			trigger_error("addDerivedProp without a name", E_USER_WARNING);
			return null;
		}

		$clsDes = $this->getClassDescriptor();
		$prop = new PntDerivedPropertyDescriptor($name, $type, $readOnly, $minValue, $maxValue, $minLength, $maxLength, $classDir);
		$clsDes->addPropertyDescriptor($prop);
		return $prop;
	}