docs: public Pair<Long, Long> getFirstGap() {
          if (m_map.isEmpty() || size() < 2) {
              return null;
          }
          Iterator<Range<Long>> iter = m_map.asRanges().iterator();
          long start = end(iter.next()) + 1;
          long end = start(iter.next()) - 1;
          return new Pair<Long, Long>(start, end);
      }