How to deal with nesting and recursion?

[arnoweiss]

There's a couple of points where the DSP allows recursive objects:

1. a Catalog can contain Catalogs in its catalog property.
2. a LogicalConstraint can contain a Constraint of which LogicalConstraint is a Subclass.

We should give implementors hints how to deal with this and what recursion depths implementations can plausibly be expected to handle.