Hmm, the following works but might look surprising:

~~~scala
other ! Request
expectOnce {
  case Response(x) if sender == other => ...
}
~~~

The surprising part is that `sender` means something different in this scope; this will be natural for newcomers and understandable for gurus but possibly a head-scratcher for intermediates. We can try it out, it is `akka-contrib` after all ;-)