How to represent annotations in serialization

[enikao]

Assuming we include annotations (#13), how to represent them in serialization format (#37)?

Aspects:

• We decided every node MUST have a parent (Repo API: Do we need model partitions? #29). What's the parent of an annotation node?
• Do we have to list languages used for the annotations in the languages header?

Alternative A: Regular containment

Change built-in uber-Concept Node to have a containment annotations: IAnnotation [0..*] that's inherited by every concept.

Same as MPS' BaseConcept.smodelAttribute

Pro:

• No special constructs needed
• Parent is inherently clear

Con:

• Need to treat this containment special in some contexts

Alternative B: Special list

In serialization, a Node currently has these lists:

• properties
• containments
• references

We could add another list annotations.

Pro:

• Clear separation of regular model contents and annotations

Con:

• The parent of an annotation would still be the annotated node. But then parent is not always the inverse of containment any more.

Alternative C: Adjacent model

(Discussed in #13, alternative G)

Decision: Alternative B: Special list