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