code: boolean addIndexedField(ReadablePeriod period, int index, int[] values, int valueToAdd) {
          if (valueToAdd == 0) {
              return false;
          }
          int realIndex = iIndices[index];
          if (realIndex == -1) {
              throw new UnsupportedOperationException("Field is not supported");
          }
          values[realIndex] = FieldUtils.safeAdd(values[realIndex], valueToAdd);
          return true;
      }
docs: Adds to the indexed field part of the period.
  
  @param period  the period to query
  @param index  the index to use
  @param values  the array to populate
  @param valueToAdd  the value to add
  @return true if the array is updated
  @throws UnsupportedOperationException if not supported