auto ym0 = YearMonth(2020, Month.jan);

ym0.add!"months"(1);
assert(ym0.year == 2020);
assert(ym0.month == Month.feb);

auto ym1 = ym0.add!"months"(1);
assert(ym1.year == 2020);
assert(ym1.month == Month.mar);

// also changes ym0
assert(ym0.year == 2020);
assert(ym0.month == Month.mar);

ym1.add!"months"(10);
assert(ym1.year == 2021);
assert(ym1.month == Month.jan);

ym1.add!"months"(-13);
assert(ym1.year == 2019);
assert(ym1.month == Month.dec);