docs: public Period withFieldAdded(DurationFieldType field, int value) {
          if (field == null) {
              throw new IllegalArgumentException("Field must not be null");
          }
          if (value == 0) {
              return this;
          }
          int[] newValues = getValues();  // cloned
          super.addFieldInto(newValues, field, value);
          return new Period(newValues, getPeriodType());
      }