docs: protected int[] addPeriodInto(int[] values, ReadablePeriod period) {
          for (int i = 0, isize = period.size(); i < isize; i++) {
              DurationFieldType type = period.getFieldType(i);
              int value = period.getValue(i);
              if (value != 0) {
                  int index = indexOf(type);
                  if (index == -1) {
                      throw new IllegalArgumentException(
                          "Period does not support field '" + type.getName() + "'");
                  } else {
                      values[index] = FieldUtils.safeAdd(getValue(index), value);
                  }
              }
          }
          return values;
      }