code: 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;
      }
docs: Adds the fields from another period.
  
  @param values  the array of values to update
  @param period  the period to add from, not null
  @return the updated values
  @throws IllegalArgumentException if an unsupported field's value is non-zero