docs: public Interval gap(ReadableInterval interval) {
          interval = DateTimeUtils.getReadableInterval(interval);
          long otherStart = interval.getStartMillis();
          long otherEnd = interval.getEndMillis();
          long thisStart = getStartMillis();
          long thisEnd = getEndMillis();
          if (thisStart > otherEnd) {
              return new Interval(otherEnd, thisStart, getChronology());
          } else if (otherStart > thisEnd) {
              return new Interval(thisEnd, otherStart, getChronology());
          } else {
              return null;
          }
      }