public void adding_an_item_updates_the_count()
{
var model = new DoubleCollectionModel();
var observer = model.CreateObserver();
Assert.Equal(0, model.Count);
model.Add(1d);
Assert.Equal(1, model.Count);
model.Add(1d);
Assert.Equal(2, model.Count);
Assert.Equal(2, observer.GetChangeCount(x => x.Count));
}