ert

A combinator to control future execution order.

```rust struct Data { file: String, tag: usize, value: usize, }

fn main() {

// Stream of `Data` coming over TCP.
let f = tcp_stream()
    .map(|d| {
        let tag = d.tag;
        Ok(d)
            .into_future()
            .and_then(move |d| read_value_from_file(&d.file).map(move |v| (v, d)))
            .and_then(move |(v, d)| write_value_to_file(&d.file, v + d.value))
            .map_err(|_| ())
            .via_g(tag) // Add execution order constrains
    })
    .buffer_unordered(100)
    .for_each(|_| Ok(()));

tokio::run(f);

} ```