/**
  * Helper to create individual filter
  * @param  string                        $fieldName
  * @param  string                        $filterString
  * @return App_ListFilter_FilterAbstract
  */
 public static function createItemFilter($fieldName, $filterString, $factory = null)
 {
     if (!$factory) {
         $factory = new self();
     }
     return $factory->_createFilter($fieldName, $filterString);
 }