code: public YearQuarter plusYears(long yearsToAdd) {
          if (yearsToAdd == 0) {
              return this;
          }
          int newYear = YEAR.checkValidIntValue(year + yearsToAdd);  // safe overflow
          return with(newYear, quarter);
      }
docs: Returns a copy of this year-quarter with the specified period in years added.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param yearsToAdd  the years to add, may be negative
  @return a {@code YearQuarter} based on this year-quarter with the years added, not null
  @throws DateTimeException if the result exceeds the supported range