public void testWithers() {
        MonthDay test = new MonthDay(10, 6);
        check(test.withMonthOfYear(5), 5, 6);
        check(test.withDayOfMonth(2), 10, 2);
        try {
            test.withMonthOfYear(0);
            fail();
        } catch (IllegalArgumentException ex) {}
        try {
            test.withMonthOfYear(13);
            fail();
        } catch (IllegalArgumentException ex) {}
    }