  /**
   * Returns a new observable that triggers on the second and subsequent triggerings of the input observable. 
   * The Nth triggering of the input observable passes the arguments from the N-1th and Nth triggering as a pair. 
   * The argument passed to the N-1th triggering is held in hidden internal state until the Nth triggering occurs.
   * @returns {Observable} An observable that triggers on successive pairs of observations from the input observable as an array.
   */
  observableProto.pairwise = function () {
    var source = this;
    return new AnonymousObservable(function (observer) {
      var previous, hasPrevious = false;
      return source.subscribe(
        function (x) {
          if (hasPrevious) {
            observer.onNext([previous, x]);
          } else {
            hasPrevious = true;
          }
          previous = x;
        },
        observer.onError.bind(observer),
        observer.onCompleted.bind(observer));
    });
  };