docs: public void add(PGInterval interval) {
      interval.setYears(interval.getYears() + getYears());
      interval.setMonths(interval.getMonths() + getMonths());
      interval.setDays(interval.getDays() + getDays());
      interval.setHours(interval.getHours() + getHours());
      interval.setMinutes(interval.getMinutes() + getMinutes());
      interval.setSeconds(interval.getSeconds() + getSeconds());
    }