Observable Scan Implementation.

TestObservables.cs

   public static IObservable<T> MyScan<T>(this IObservable<T> stream, T init, Func<(T acc, T curr), T> resultCalc) 
   {
      return Observable.Create<T>(obs => 
      {
        var result = init;
        var dispose = 
            stream.Subscribe( 
                x => {
                    result = resultCalc((result, x));
                    obs.OnNext(result);
                },
                obs.OnError,
                obs.OnComplete
            );
        return() => { dispose.Dispose(); };
    });