protected async Task<List<string>> DownloadUrls(string url)
        {
            var req = HttpWebRequest.CreateHttp(url);
            var res = (HttpWebResponse)req.GetResponse();
            if (!res.IsHtmlPage() || res.StatusCode != HttpStatusCode.OK) {
                return new List<string>();
            }
            var content = await Task.Factory.StartNew(() => req.GetResponse().Download());
            return await Task.Factory.StartNew(() => {
                var dom = CsQuery.CQ.CreateDocument(content);
                return dom["a"]
                    .Select(l => MakeAbsolute(l.GetAttribute("href"), this.BaseUrl))
                    .ToList();
            });
        }

        protected async Task<List<string>[]> DownloadAsync(List<string> urls)
        {
            //Logger.Log(String.Join("\n", urls), LoggerMessageType.Info);
            using (var semaphore = new SemaphoreSlim(5)) {
                var tasks = urls.Select(async (url) => {
                    await semaphore.WaitAsync();
                    try {
                        var fetchedUrls = await DownloadUrls(url);
                        return fetchedUrls;
                    } finally {
                        semaphore.Release();
                    }
                });
                return await Task.WhenAll(tasks);
            }
        }

        protected async void RecursiveParse(string baseHost)
        {
            var urls = await DownloadAsync(new List<string> { baseHost });
            foreach (var url in urls) {
                RecursiveParse(url, 3 /* depth of recursion */);
            }
        }

        protected async void RecursiveParse(List<string> urls, int depth)
        {
            if (depth <= 0) {
                return;
            }
            var newLinks = await DownloadAsync(urls);
            var y = newLinks.SelectMany(x => x).Distinct().ToList();
            RecursiveParse(y, depth - 1);
        }