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