public static IObservable <CollectionAddRemoveEvent <T> > ObserveCurrentAddRemove <T>(this IReadOnlyReactiveCollection <T> This) =>
This.Select((x, i) => new CollectionAddRemoveEvent <T>(i, x, true))
.ToObservable()
.Concat(This
.ObserveAdd()
.Select(x => new CollectionAddRemoveEvent <T>(x.Index, x.Value, true))
.Merge(This
.ObserveRemove()
.Select(x => new CollectionAddRemoveEvent <T>(x.Index, x.Value, false))));
