   /**
   *  Determines whether an observable collection contains values. 
   *
   * @example
   *  1 - res = Rx.Observable.if(condition, obs1);
   *  2 - res = Rx.Observable.if(condition, obs1, obs2);
   *  3 - res = Rx.Observable.if(condition, obs1, scheduler);
   * @param {Function} condition The condition which determines if the thenSource or elseSource will be run.
   * @param {Observable} thenSource The observable sequence or Promise that will be run if the condition function returns true.
   * @param {Observable} [elseSource] The observable sequence or Promise that will be run if the condition function returns false. If this is not provided, it defaults to Rx.Observabe.Empty with the specified scheduler.
   * @returns {Observable} An observable sequence which is either the thenSource or elseSource.
   */
  Observable['if'] = function (condition, thenSource, elseSourceOrScheduler) {
    return observableDefer(function () {
      elseSourceOrScheduler || (elseSourceOrScheduler = observableEmpty());

      isPromise(thenSource) && (thenSource = observableFromPromise(thenSource));
      isPromise(elseSourceOrScheduler) && (elseSourceOrScheduler = observableFromPromise(elseSourceOrScheduler));

      // Assume a scheduler for empty only
      typeof elseSourceOrScheduler.now === 'function' && (elseSourceOrScheduler = observableEmpty(elseSourceOrScheduler));
      return condition() ? thenSource : elseSourceOrScheduler;
    });
  };
