fun <T, R> ReceiveChannel<T>.flatMap(context: CoroutineContext = Unconfined, onError: (Throwable) -> Unit = logOnError, mapper: suspend (T) -> ReceiveChannel<R>): ReceiveChannel<R> = produce<R>(context) {
    try {
        for (elem in this@flatMap)
            for (mapped in mapper(elem))
                send(mapped)
    } catch (t: Throwable) {
        onError(t)
    }
}

flatMap