public void testFormatAppend_PrinterParser_Printer_null_null_Parser() {
        PeriodPrinter printer = new PeriodFormatterBuilder().appendYears().appendLiteral("-").toPrinter();
        PeriodParser parser = new PeriodFormatterBuilder().appendWeeks().appendLiteral("-").toParser();
        PeriodFormatterBuilder bld = new PeriodFormatterBuilder().append(printer, null).append(null, parser);
        assertNull(bld.toPrinter());
        assertNull(bld.toParser());
        
        try {
            bld.toFormatter();
            fail();
        } catch (IllegalStateException ex) {}
    }