/**
   * Transforms an observable sequence of observable sequences into an observable sequence producing values only from the most recent observable sequence.
   * @returns {Observable} The observable sequence that at any point in time produces the elements of the most recent inner observable sequence that has been received.
   */
  observableProto['switch'] = observableProto.switchLatest = function () {
    var sources = this;
    return new AnonymousObservable(function (observer) {
      var hasLatest = false,
        innerSubscription = new SerialDisposable(),
        isStopped = false,
        latest = 0,
        subscription = sources.subscribe(
          function (innerSource) {
            var d = new SingleAssignmentDisposable(), id = ++latest;
            hasLatest = true;
            innerSubscription.setDisposable(d);

            // Check if Promise or Observable
            isPromise(innerSource) && (innerSource = observableFromPromise(innerSource));

            d.setDisposable(innerSource.subscribe(
              function (x) { latest === id && observer.onNext(x); },
              function (e) { latest === id && observer.onError(e); },
              function () {
                if (latest === id) {
                  hasLatest = false;
                  isStopped && observer.onCompleted();
                }
              }));
          },
          observer.onError.bind(observer),
          function () {
            isStopped = true;
            !hasLatest && observer.onCompleted();
          });
      return new CompositeDisposable(subscription, innerSubscription);
    }, sources);
  };