code: public YearWeek plusWeeks(long weeksToAdd) {
          if (weeksToAdd == 0) {
              return this;
          }
          LocalDate mondayOfWeek = atDay(DayOfWeek.MONDAY).plusWeeks(weeksToAdd);
          return YearWeek.from(mondayOfWeek);
      }
docs: Returns a copy of this year-week with the specified number of weeks added.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param weeksToAdd  the weeks to add, may be negative
  @return the year-week with the weeks added, not null