auto ym = YearMonth(2000, Month.nov);
assert(ym.opCmp(YearMonth(2000, Month.nov)) == 0);
assert(ym.opCmp(YearMonth(2000, Month.oct)) == 1);
assert(ym.opCmp(YearMonth(2000, Month.dec)) == -1);
assert(ym.opCmp(YearMonth(2001, Month.nov)) == -1);