I looked up Date's two exceptions. The first is uninteresting---databases cannot contain pointers---but the second is sort of relevant: > If a relation r has h

I looked up Date's two exceptions. The first is uninteresting---databases cannot contain pointers---but the second is sort of relevant: > If a relation r has heading H, then no attribute of r can be defined in terms of a relation or tuple type that has the same heading H, at any level of nesting. In other words, relations can nest, but not recurse.