public static function getOutputFormatter( $format )
    {
        if ( !self::isRegisteredFormatter( $format ) )
        {
            $format = eZINI::instance( 'template.ini' )->variable( 'AttributeOperator', 'DefaultFormatter' );
        }

        if ( !( self::$formatter instanceof ezpAttributeOperatorFormatterInterface ) || $format != self::$format )
        {
            self::$formatter = self::createFormatter( $format );
            self::$format = $format;
        }

        return self::$formatter;
    }