public async void RefreshTopStories()
        {
            if (!isUpdating.GetAndSet(true))
            {
                NotifyOfPropertyChange("IsUpdating");

                try
                {
                    var newStories = await hn.GetTopStoriesAsync();

                    var existingIds = new HashSet <long>(topStories.Select(ts => ts.Id));
                    var added       = false;

                    // assuming that story IDs are chronologically-ordered
                    foreach (var story in from ns in newStories
                             where !existingIds.Contains(ns.Id)
                             orderby ns.Id ascending
                             select new TopStoryViewModel(ns, navigationService))
                    {
                        added = true;
                        topStories.Insert(0, story);
                    }

                    if (added && existingIds.Count > 0)
                    {
                        // need to sort the list
                        var items = topStories.OrderBy(ts => ts.Id).ToList();
                        topStories.Clear();
                        foreach (var ts in items)
                        {
                            topStories.Add(ts);
                        }
                    }
                }
                catch (Exception ex)
                {
                    // whoops
                    Log.Error(ex);
                }
                finally
                {
                    IsUpdating = false;
                }
            }
        }