code: @Nullable
    public Document getDocument (final int nDocID) throws IOException
    {
      _checkClosing ();
  
      if (LOGGER.isDebugEnabled ())
        LOGGER.debug ("getDocument(" + nDocID + ")");
  
      final IndexReader aReader = getReader ();
      if (aReader == null)
        return null;
      return aReader.document (nDocID);
    }
docs: Get the Lucene {@link Document} matching the specified ID
  
  @param nDocID
  Document ID
  @return <code>null</code> if no reader could be obtained or no such
  document exists.
  @throws IOException
  On IO error