[xacml-comment] subject-category as attribute, rather than <Attribute>

I can see why the subject-category attribute has been modelled the
way that it has, as an <Attribute> of the <Subject> element. But
how about modelling it as an XML attribute of the <Subject>
element instead?

<Subject Category='...:access-subject'>...</Subject>

This would enforce the constraint that there be only one subject-
category attribute in the XML Schema. This would also make
implementing a request processor a little simpler. And, I think
would make understanding this feature of the standard simpler.

John