code: ValueIndex lookup(IndexReader reader, String field, String prefix)
     {
        synchronized (cache)
        {
           Map<Key, ValueIndex> readerCache = cache.get(reader);
           if (readerCache == null)
           {
              return null;
           }
           return readerCache.get(new Key(field, prefix));
        }
     }
docs: See if a <code>ValueIndex</code> object is in the cache.