   /**
   *  Uses selector to determine which source in sources to use.
   *  There is an alias 'switchCase' for browsers <IE9.
   *  
   * @example
   *  1 - res = Rx.Observable.case(selector, { '1': obs1, '2': obs2 });
   *  1 - res = Rx.Observable.case(selector, { '1': obs1, '2': obs2 }, obs0);
   *  1 - res = Rx.Observable.case(selector, { '1': obs1, '2': obs2 }, scheduler);
   * 
   * @param {Function} selector The function which extracts the value for to test in a case statement.
   * @param {Array} sources A object which has keys which correspond to the case statement labels.
   * @param {Observable} [elseSource] The observable sequence or Promise that will be run if the sources are not matched. If this is not provided, it defaults to Rx.Observabe.empty with the specified scheduler.
   *       
   * @returns {Observable} An observable sequence which is determined by a case statement.  
   */
  Observable['case'] = Observable.switchCase = function (selector, sources, defaultSourceOrScheduler) {
    return observableDefer(function () {
      isPromise(defaultSourceOrScheduler) && (defaultSourceOrScheduler = observableFromPromise(defaultSourceOrScheduler));
      defaultSourceOrScheduler || (defaultSourceOrScheduler = observableEmpty());

      typeof defaultSourceOrScheduler.now === 'function' && (defaultSourceOrScheduler = observableEmpty(defaultSourceOrScheduler));
      
      var result = sources[selector()];
      isPromise(result) && (result = observableFromPromise(result));
      
      return result || defaultSourceOrScheduler;
    });
  };
