Use service response as message type

edit

I have two services. The first one calculates some stuff and the other one is supposed to generate additional data based on the former's response. The wiki states that both request and response are msg types, so they should be usable as service fields like this:

A.srv

int64 x
---
int64[] y
float32 z

B.srv

AResponse a_res
---
string comment

This does not seem to work however. I would very much like to avoid keeping request and response of two services in sync. Is this even possible, and if so, what did I overlook?