code: void updateDeletedDocs(CommittableIndexReader reader)
     {
        int maxDoc = reader.maxDoc();
        for (int i = 0; i < maxDoc; i++)
        {
           if (reader.isDeleted(i))
           {
              deleted.set(i);
           }
        }
        deletedDocsVersion = reader.getModificationCount();
     }
docs: Updates the deleted documents in this index reader. When this method
  returns this index reader will have the same documents marked as deleted
  as the passed <code>reader</code>.
  <br>
  This method is not thread-safe! Make sure no other thread is concurrently
  using this reader at the same time.
  
  @param reader the reader from where to obtain the deleted documents
  info.