/**
   * Applies an accumulator function over an observable sequence, returning the result of the aggregation as a single element in the result sequence. The specified seed value is used as the initial accumulator value.
   * For aggregation behavior with incremental intermediate results, see Observable.scan.
   * @param {Function} accumulator An accumulator function to be invoked on each element.
   * @param {Any} [seed] The initial accumulator value.
   * @returns {Observable} An observable sequence containing a single element with the final accumulator value.
   */
  observableProto.reduce = function (accumulator) {
    var hasSeed = false, seed, source = this;
    if (arguments.length === 2) {
      hasSeed = true;
      seed = arguments[1];
    }
    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;
          }
        },
        function (e) { o.onError(e); },
        function () {
          hasValue && o.onNext(accumulation);
          !hasValue && hasSeed && o.onNext(seed);
          !hasValue && !hasSeed && o.onError(new Error(sequenceContainsNoElements));
          o.onCompleted();
        }
      );
    }, source);
  };