internal static void Close(IndexReader reader)
        {
            lock (Comparators.SyncRoot)
            {
                System.Collections.Hashtable readerCache = (System.Collections.Hashtable)Comparators[reader];
                if (readerCache != null)
                {
                    readerCache.Clear();
                    readerCache = null;
                }

                Comparators.Remove(reader);
            }
        }