@OptIn(ExperimentalCoroutinesApi::class, FlowPreview::class)
fun <A, B : Any, R> Flow<A>.doThing(
	flow: Flow<B?>,
	transform: suspend (A, B?) -> R
): Flow<R> {
    return flow {
        coroutineScope {
            val aChannel = this@doThing.produceIn(this)
            val bChannel = flow.produceIn(this)

            var lastA: A = aChannel.receive()
            emit(transform(lastA, bChannel.tryReceive().getOrNull()))

            whileSelect {
                aChannel.onReceive { a ->
                    emit(transform(a, null))
                    lastA = a
                    true
                }
                bChannel.onReceive { b ->
                    emit(transform(lastA, b))
                    true
                }
            }
        }
    }
}