code: public ZipFileData addLast(ZipFileData newLastData) {
          String newLastPath = newLastData.path;
  
          Cell dupCell = cells.get(newLastPath);
          if ( dupCell != null ) {
              throw new IllegalArgumentException("Path [ " + newLastPath + " ] is already stored");
          }
  
          Cell oldLastCell = anchor.prev;
  
          Cell newLastCell = new Cell(newLastData);
          cells.put(newLastPath, newLastCell);
  
          newLastCell.putBetween(oldLastCell, anchor);
  
          return oldLastCell.data;
      }
docs: Add data as new last data.
  
  Answer the previous last data.
  
  Throw an exception if the path of the new data is already stored.
  
  @param newLastData The new data to put as the last stored data.
  
  @return The data which was previously the last stored data.  Null
  if the store was empty.