auto yq0 = YearQuarter(2020, Quarter.q1);

yq0.add!"years"(1);
assert(yq0.year == 2021);
assert(yq0.quarter == Quarter.q1);

auto yq1 = yq0.add!"years"(1);
assert(yq1.year == 2022);
assert(yq1.quarter == Quarter.q1);

// also changes yq0
assert(yq0.year == 2022);
assert(yq0.quarter == Quarter.q1);