code: void store(IndexReader reader, String field, String prefix, ValueIndex value)
     {
        synchronized (cache)
        {
           Map<Key, ValueIndex> readerCache = cache.get(reader);
           if (readerCache == null)
           {
              readerCache = new HashMap<Key, ValueIndex>();
              cache.put(reader, readerCache);
           }
           readerCache.put(new Key(field, prefix), value);
        }
     }
docs: Put a <code>ValueIndex</code> <code>value</code> to cache.