Base.cs

var items = CreateRequestUris(requestXml).
    Select(Downloader.Download).
    SelectMany(Parse);
return new Response(TransformResult(items));

ReactiveImpl.cs

 readonly ReplaySubject<string> urlSubject = new ReplaySubject<string>();

 protected override IEnumerable<string> CreateRequestUris(XmlElement requestXml)
 {
     urlSubject.OnNext(ListUrl);
     return urlSubject.ToEnumerable();
 }

 protected override IEnumerable<Data> Parse(string text)
 {
     var html = text.ParseHtml();
     NotifyNextPage(html);
     // ...
     return from row in rows select Data.Parse(row);
}

void NotifyNextPage(XElement html)
{
    var formData = ParseFormData(html);
    if (formData != null)
        urlSubject.OnNext(ListUrl + formData);
    else
        urlSubject.OnCompleted();
}