XMLSchema validation during an Update

["Brian Vickery" <macacos@linkline.com>]

How would an XMLSchema be used to validate an update elements/attributes/
and/or values during an XUpdate XPath expression?  Since an XMLSchema is
associated with a "document" and not a fragment of a document, does this
imply one must retrieve the complete document and not just the fragment to
do validation before actually updating the fragment?  This could be
QUITE expensive. 

thanks,