openAtEnd should raise an error on any element that doesn't repeat (as there is no notion of 'end' for polymorphic resources). openAtEnd should raise at least a warning on repeating elements if orderMeaning is null because forcing elements to appear in a particular order when order has no meaning breaks interoperability unnecessarily.

We should also spit out an information message for 'closed' slices advising that closing slices may increase costs by preventing implementers from constructing instances that satisfy multiple profiles and that ignoring unexpected slices may be preferable to rejecting them.