   @warn_unused_result
   public static func never() -> Stream<Element> {
     return Stream { observer in
-      return SimpleDisposable()
+      return NotDisposable
     }
   }
 

  public static func completed() -> Stream<Element> {
    return Stream { observer in
      observer.completed()
      return SimpleDisposable()
    }
  }


    return Stream { observer in
      sequence.forEach(observer.next)
      observer.completed()
      return NotDisposable
    }
  }

