@Override
  public OperationFuture<Boolean> prepend(String key, Object val) {
    return prepend(0, key, val, transcoder);
  }
Prepend to an existing value in the cache.

<p>
Note that the return will be false any time a mutation has not occurred.
</p>

@param key the key to whose value will be prepended
@param val the value to append
@return a future indicating success
@throws IllegalStateException in the rare circumstance where queue is too
          full to accept any more requests
src/main/java/net/spy/memcached/MemcachedClient.java