So I tried this out preparing the next patch it doesn't... *feel* right to have a *ReadOnly* flag in each *Attribute* structure. I'm going to defer immutability until the next patch. Perhaps it would be better to have immutability set on *Instances* (and add a pointer to an active *Instances* set to each *Attribute*) or drop immutability (given that we'll never be able to completely enforce it without language support).