future<void> exchange_async(/* ... */)
{
    vector<future<void> > outbound = /* ... */;
    vector<future<void> > inbound  = /* ... */;

    future<void> when_out = when_all(outbound);
    future<void> when_in  = when_all(inbound);
    return when_all(when_out, when_in);
}