from reactive programms edx course

// Utilities to temporarily have unimplemented parts of the program

   private def unimplementedFlow[A, B, C]: Flow[A, B, C] =
    Flow.fromFunction[A, B](_ => ???).mapMaterializedValue(_ => ??? : C)

  private def unimplementedSink[A, B]: Sink[A, B] = Sink.ignore.mapMaterializedValue(_ => ??? : B)