code: @Override
    public IScan getScanByNumLower(int scanNum) {
      IScan scan = getNum2scan().lowerEntry(scanNum).getValue();
      if (scan != null) {
        return scan;
      }
      return null;
    }
docs: Scan with closest Number STRICTLY less than the provided one is returned.
  
  @param scanNum scan number
  @return Scan or null, if the ScanCollection didn't have any scans with numbers <= than this one