code: public YearQuarter plusQuarters(long quartersToAdd) {
          if (quartersToAdd == 0) {
              return this;
          }
          long quarterCount = year * 4L + (quarter.getValue() - 1);
          long calcQuarters = quarterCount + quartersToAdd;  // safe overflow
          int newYear = YEAR.checkValidIntValue(Math.floorDiv(calcQuarters, 4));
          int newQuarter = (int) Math.floorMod(calcQuarters, 4) + 1;
          return with(newYear, Quarter.of(newQuarter));
      }
docs: Returns a copy of this year-quarter with the specified period in quarters added.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param quartersToAdd  the quarters to add, may be negative
  @return a {@code YearQuarter} based on this year-quarter with the quarters added, not null
  @throws DateTimeException if the result exceeds the supported range