fn split_sender<LeftData: Data, RightData: Data, S: Sender<Split<LeftData, RightData>>>(
    sender: S
) -> (SplitSender<LeftData, RightData, S, ToLeftSplitConvert<LeftData, RightData>>, SplitSender<LeftData, RightData, S, ToRightSplitConvert<LeftData, RightData>>)