/// <summary>
 /// Calling this method will establish the asynchronous callback used when the request receives a response.
 /// </summary>
 /// <param name="instance">The current location in the fluent expression</param>
 /// <param name="callback">The callback executed when a request completes in the background</param>
 /// <returns>The current location in the fluent expression</returns>
 public static IFluentTwitter CallbackTo(this IFluentTwitter instance, TwitterWebCallback callback)
     instance.Callback = callback;
     return instance;
 public ITwitterLeafNode CallbackTo(TwitterWebCallback callback)
     return this;