code: @Override
    public NavigableMap<Integer, IScan> getScansByNumSpanAtMsLevel(int numStart, int numEnd,
        int msLevel) {
      NavigableMap<Integer, IScan> subMap = null;
      TreeMap<Integer, IScan> msLevelMap = getMapMsLevel2index().get(msLevel).getNum2scan();
      if (msLevelMap != null) {
        subMap = msLevelMap.subMap(numStart, true, numEnd, true);
      }
      if (subMap != null && subMap.size() > 0) {
        return subMap;
      }
      return null;
    }
docs: A view(not a copy!) of the original scan map, containing only scans from numStart to numEnd
  (inclusive). Scan numbers don't need to be exact matches, scans from the closed interval
  [numStart; numEnd] are returned.
  
  @return A view or null if there were no scans at this ms level in [numStart; numEnd] interval