docs: public long add(ReadablePeriod period, long instant, int scalar) {
          if (scalar != 0 && period != null) {
              for (int i = 0, isize = period.size(); i < isize; i++) {
                  long value = period.getValue(i); // use long to allow for multiplication (fits OK)
                  if (value != 0) {
                      instant = period.getFieldType(i).getField(this).add(instant, value * scalar);
                  }
              }
          }
          return instant;
      }