A combinator to control future execution order.
```rust struct Data { file: String, tag: usize, value: usize, }
async fn main() {
// Stream of Data coming over TCP.
let f = tcpstream()
.map(|d| {
let tag = d.tag;
let f = async move {
let v = readvaluefromfile(&d.file).await;
writevaluetofile(&d.file, v + d.value).await;
};
f.viag(tag)
})
.bufferunordered(100)
.foreach(|_| async {});
f.await;
} ```
