XMI13 — More use of schema structural constraints

It is not clear why a separate XML construct has not been used for proxies
(as described in 4.9) - rather than leaving an importer to check for the
presence of link attributes. And providing no Schema-level enforceable
constraint that proxies have no content (and vice versa).
It would also help to have a stronger statement with respect to the use of
XMI attributes as a 'cache' within proxies: to what extent must they be
accurate and what options does an importer have if they are not? Does
validation include checking the accuracy of cached attributes?
4.6.2: Could Schema mechanisms be used to enforce the constraint that only
one of href and idref is to be used?