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

yq0.add!"quarters"(1);
assert(yq0.year == 2020);
assert(yq0.quarter == Quarter.q2);

auto yq1 = yq0.add!"quarters"(1);
assert(yq1.year == 2020);
assert(yq1.quarter == Quarter.q3);

// also changes yq0
assert(yq0.year == 2020);
assert(yq0.quarter == Quarter.q3);

yq1.add!"quarters"(2);
assert(yq1.year == 2021);
assert(yq1.quarter == Quarter.q1);

yq1.add!"quarters"(-5);
assert(yq1.year == 2019);
assert(yq1.quarter == Quarter.q4);