//set the value of the property with the specified name
	// The value is pased by value, thus copied.
	// If you need to pass the value by reference, use
	// PntPropertyDescriptor::setValue_for directly
	function set($propertyName, $value) {
		$prop = $this->getPropertyDescriptor($propertyName);
		if (!$prop) {
			trigger_error(
				$this->getClass()." property not found: $propertyName"
				, E_USER_ERROR);
			return null;
		}
		$prop->setValue_for($value, $this);
		return true;
	}