protected IObservable<T> OpenXmlReader<T>(string url,
  Func<XmlReader, IEnumerable<T>> readerFunc)
{
  var req = HttpWebRequest.Create(url);
  return Observable
    .FromAsyncPattern<WebResponse>(req.BeginGetResponse, req.EndGetResponse)
    .Invoke()
    .SelectMany(resp => Observable
      .Using(() => resp, r => Observable // using (response)
        .Using(() => r.GetResponseStream(), stream => Observable // using (stream)
          .Return(createReader(stream))
          .SelectMany(readerFunc))));
}