  var Enumerable = Rx.internals.Enumerable = function (iterator) {
    this._iterator = iterator;
  };

  Enumerable.prototype[$iterator$] = function () {
    return this._iterator();
  };

  Enumerable.prototype.concat = function () {
    var sources = this;
    return new AnonymousObservable(function (observer) {
      var e;
      try {
        e = sources[$iterator$]();
      } catch(err) {
        observer.onError();
        return;
      }

      var isDisposed, 
        subscription = new SerialDisposable();
      var cancelable = immediateScheduler.scheduleRecursive(function (self) {
        var currentItem;
        if (isDisposed) { return; }

        try {
          currentItem = e.next();
        } catch (ex) {
          observer.onError(ex);
          return;
        }

        if (currentItem.done) {
          observer.onCompleted();
          return;
        }

        // Check if promise
        var currentValue = currentItem.value;
        isPromise(currentValue) && (currentValue = observableFromPromise(currentValue));

        var d = new SingleAssignmentDisposable();
        subscription.setDisposable(d);
        d.setDisposable(currentValue.subscribe(
          observer.onNext.bind(observer),
          observer.onError.bind(observer),
          function () { self(); })
        );
      });

      return new CompositeDisposable(subscription, cancelable, disposableCreate(function () {
        isDisposed = true;
      }));
    });
  };

  Enumerable.prototype.catchException = function () {
    var sources = this;
    return new AnonymousObservable(function (observer) {
      var e;
      try {
        e = sources[$iterator$]();
      } catch(err) {
        observer.onError();
        return;
      }

      var isDisposed, 
        lastException,
        subscription = new SerialDisposable();
      var cancelable = immediateScheduler.scheduleRecursive(function (self) {
        if (isDisposed) { return; }

        var currentItem;
        try {
          currentItem = e.next();
        } catch (ex) {
          observer.onError(ex);
          return;
        }

        if (currentItem.done) {
          if (lastException) {
            observer.onError(lastException);
          } else {
            observer.onCompleted();
          }
          return;
        }

        // Check if promise
        var currentValue = currentItem.value;
        isPromise(currentValue) && (currentValue = observableFromPromise(currentValue));        

        var d = new SingleAssignmentDisposable();
        subscription.setDisposable(d);
        d.setDisposable(currentValue.subscribe(
          observer.onNext.bind(observer),
          function (exn) {
            lastException = exn;
            self();
          },
          observer.onCompleted.bind(observer)));
      });
      return new CompositeDisposable(subscription, cancelable, disposableCreate(function () {
        isDisposed = true;
      }));
    });
  };

  var enumerableRepeat = Enumerable.repeat = function (value, repeatCount) {
    if (repeatCount == null) { repeatCount = -1; }
    return new Enumerable(function () {
      var left = repeatCount;
      return new Enumerator(function () {
        if (left === 0) { return doneEnumerator; }
        if (left > 0) { left--; }
        return { done: false, value: value };
      });
    });
  };

  var enumerableFor = Enumerable.forEach = function (source, selector, thisArg) {
    selector || (selector = identity);
    return new Enumerable(function () {
      var index = -1;
      return new Enumerator(
        function () {
          return ++index < source.length ?
            { done: false, value: selector.call(thisArg, source[index], index, source) } :
            doneEnumerator;
        });
    });
  };
