  /**
   *  Records the timestamp for each value in an observable sequence.
   *  
   * @example
   *  1 - res = source.timestamp(); // produces { value: x, timestamp: ts }
   *  2 - res = source.timestamp(Rx.Scheduler.timeout);
   *      
   * @param {Scheduler} [scheduler]  Scheduler used to compute timestamps. If not specified, the timeout scheduler is used.
   * @returns {Observable} An observable sequence with timestamp information on values.
   */
  observableProto.timestamp = function (scheduler) {
    isScheduler(scheduler) || (scheduler = timeoutScheduler);
    return this.map(function (x) {
      return { value: x, timestamp: scheduler.now() };
    });
  };
