Rx delta without Zip/DualSubscription

PairWithPrevious.cs

//From James World - http://social.msdn.microsoft.com/Forums/en-US/dfa87af8-b7dd-4fb5-abe2-99348d3e27e1/compare-two-object-graphs-and-get-differences?forum=rx
public static IObservable<Tuple<TSource, TSource>> PairWithPrevious<TSource>(this IObservable<TSource> source)
{
    return source.Scan(
        Tuple.Create(default(TSource), default(TSource)),
        (acc, current) => Tuple.Create(acc.Item2, current));
}