docs: public long set(long instant, int era) {
          FieldUtils.verifyValueBounds(this, era, DateTimeConstants.BCE, DateTimeConstants.CE);
              
          int oldEra = get(instant);
          if (oldEra != era) {
              int year = iChronology.getYear(instant);
              return iChronology.setYear(instant, -year);
          } else {
              return instant;
          }
      }