public override IObservable <CrawlResult> Crawl(CrawlResult result)
{
return(Observable.If(() => Extractor.CanExtract(result),
Extractor.Extract(result).ToObservable()
.Do(x => Console.WriteLine("Crawling: " + x))
.SelectMany(uri => base.Crawl(new CrawlResult(uri, null)))));
}