/** Validate the value for the property. Answer null if valid.
* Answer the error String if invalid.
* Override this method to modify the validation behavior.
* Default is just to use the validator from _getValueValidator.
* You may change ValueValidator to override the error messages
* or the generic behavior inherited from PntValueValidator
* @param prop PntPropertyDescriptor
* @param value mixed value to be validated.
* @return String the error string or null if the value is valid
*/
function (&$prop, $value) {
$validator =& $this->($prop);
return $validator->($value);
}