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

ym0.add!"years"(1);
assert(ym0.year == 2021);
assert(ym0.month == Month.jan);

auto ym1 = ym0.add!"years"(1);
assert(ym1.year == 2022);
assert(ym1.month == Month.jan);

// also changes ym0
assert(ym0.year == 2022);
assert(ym0.month == Month.jan);