Jeroen,
As far as I've seen from the models that were presented at the NML meetings, the way you describe adaptation/services very much determines the way you describe layers.Once you have defined adaptations, then the definition of layers should follow naturally.
To follow the 'building blocks' analogy: once you have determined the form of the blocks, then defining layers is just putting them together in the right way. And the form of the blocks very much determines how you are going to do that.
I very much agree with this and I think that it underlies part of our differing views on the delineation between deliverables 2 and 3. UNIS was always multi-layer and we took advantage of the namespacing technique from the NM-WG measurement stuff to accomplish that. I think that's why I see the support for the multi-layering as being present in the base, with the details of the layers themselves being in deliverable 3. martin