I think we want to define a new API to capture this.  Service types will
declare which other service types they extend and how; service types
that can be extended will declare how extensions affect their
parameters.  (We need to distinguish between “service type” and “service
instance” because there can be several instances of a given type.)

Building the system works in two steps: first we propagate the
extensions down to the sinks of the DAG (such as dmd), then we
instantiate those.

Thoughts?

I’ll try to refine this and come up with a proof-of-concept for
discussion soonish.

Thanks,
Ludo’.