code: public CalendarQuarter plus(Quarters quarters) {
  
          if (quarters.isEmpty()) {
              return this;
          }
  
          long value = this.year * 4L + this.quarter.getValue() - 1 + quarters.getAmount();
          int y = MathUtils.safeCast(MathUtils.floorDivide(value, 4));
          Quarter q = Quarter.valueOf(MathUtils.floorModulo(value, 4) + 1);
          return CalendarQuarter.of(y, q);
  
      }
docs: /*[deutsch]
  <p>Addiert die angegebenen Quartale zu diesem Kalenderquartal. </p>
  
  @param   quarters       the count of quarter years to be added
  @return  result of addition