node identity in XQuery

Hi. 

I tried some XQuery implementations(Kawa, MS XQuery Demo).
But, node identity function (==, !==) has not been implemented yet.

Is it difficult to implement node identity in XQuery data model?

I think we need unique node identifiers for all nodes in large XML
Repository.
And, it's difficult to identify all nodes efficiently if there happened
update of XML Repository.


Takeharu Eda