code: public YearMonth withMonthOfYear(int monthOfYear) {
          int[] newValues = getValues();
          newValues = getChronology().monthOfYear().set(this, MONTH_OF_YEAR, newValues, monthOfYear);
          return new YearMonth(this, newValues);
      }
docs: Returns a copy of this year-month with the month of year field updated.
  <p>
  YearMonth is immutable, so there are no set methods.
  Instead, this method returns a new instance with the value of
  month of year changed.
  
  @param monthOfYear  the month of year to set
  @return a copy of this object with the field set, never null
  @throws IllegalArgumentException if the value is invalid