protected static function singleton(GatewayType $gateway_adapter, Gateway_Extras_CustomFilters $custom_filter_object)
 {
     if (!self::$instance || $gateway_adapter->isBatchProcessor()) {
         self::$instance = new self($gateway_adapter, $custom_filter_object);
     }
     return self::$instance;
 }
 static function singleton(&$gateway_adapter, &$custom_filter_object)
 {
     if (!self::$instance || $gateway_adapter->isBatchProcessor()) {
         self::$instance = new self($gateway_adapter, $custom_filter_object);
     }
     return self::$instance;
 }