/**
   *  Applies an accumulator function over an observable sequence and returns each intermediate result. The optional seed value is used as the initial accumulator value.
   *  For aggregation behavior with no intermediate results, see Observable.aggregate.
   * @example
   *  var res = source.scan(function (acc, x) { return acc + x; });
   *  var res = source.scan(0, function (acc, x) { return acc + x; });
   * @param {Mixed} [seed] The initial accumulator value.
   * @param {Function} accumulator An accumulator function to be invoked on each element.
   * @returns {Observable} An observable sequence containing the accumulated values.
   */
  observableProto.scan = function () {
    var hasSeed = false, seed, accumulator, source = this;
    if (arguments.length === 2) {
      hasSeed = true;
      seed = arguments[0];
      accumulator = arguments[1];
    } else {
      accumulator = arguments[0];
    }
    return new AnonymousObservable(function (o) {
      var hasAccumulation, accumulation, hasValue;
      return source.subscribe (
        function (x) {
          !hasValue && (hasValue = true);
          try {
            if (hasAccumulation) {
              accumulation = accumulator(accumulation, x);
            } else {
              accumulation = hasSeed ? accumulator(seed, x) : x;
              hasAccumulation = true;
            }
          } catch (e) {
            o.onError(e);
            return;
          }

          o.onNext(accumulation);
        },
        function (e) { o.onError(e); },
        function () {
          !hasValue && hasSeed && o.onNext(seed);
          o.onCompleted();
        }
      );
    }, source);
  };