Section 4.1.2.3 in the editor's draft states
4.1.2.3 Null References
A model validator MUST not perform the SML validation associated with references for a null reference element.


This needs to be changed since we agreed to enforce targetRequired constraints on null references. The simplest soln may be to simply remove this section and replace it with the table in Sandy's proposal (subject to consensus on whether targetElement and targetType constraints need to be enforced on null references)