[xacml-comment] Section 6.2

Section 6.2 says that every <Subject> element MUST contain
one and only one <Attribute> with AttributeId
"urn:oasis:names:tc:xacml:1.0:subject:subject-category".

I'm wondering if this contradicts another sentence, which starts with
"If this attribute is not present in a given <Subject> element"

The former means that there must be a single attribute representing the
subject category.
On the other hand, the latter means that it's optional.

Is this okay?

Satoshi Hada
IBM Tokyo Research Laboratory
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