code: public static YearMonth of(int year, int month) {
          YEAR.checkValidValue(year);
          MONTH_OF_YEAR.checkValidValue(month);
          return new YearMonth(year, month);
      }
docs: Obtains an instance of {@code YearMonth} from a year and month.
  
  @param year  the year to represent, from MIN_YEAR to MAX_YEAR
  @param month  the month-of-year to represent, from 1 (January) to 12 (December)
  @return the year-month, not null
  @throws DateTimeException if either field value is invalid