key
keyref
unique
A set of [annotations] if it exists, otherwise an empty XSObjectList.
XSObjectList
[identity-constraint category]: one of key, keyref or unique.
[fields]: a non-empty list of restricted XPath 1.0 expressions.
[referenced key]: required if [identity-constraint category] is keyref, null otherwise. An identity-constraint definition with [ identity-constraint category] equal to key or unique.
null
[selector]: a restricted XPath 1.0 expression.