XUpdate: how to replace a node?

Hi, I need to replace a given node in my database by a whole new node. 
First, is the XUpdate documentation found on 
http://xmldb-org.sourceforge.net/xupdate/xupdate-wd.html current
enough?

Well, this documentation doesn't mention any replace functionality. So
I thought that I could just remove the node, and add the new node to
the parent, at the same position. Problem: in order to be able to
append the new node to the parent, I need to put a reference to the
parent in a variable before deleting the old node: my XPath to the
node is like //node[@id="2"], so I can easily find the parent before
deleting the node, but it is impossible afterwards.

The problem is that I cannot define variables in xupdate:
<xu:variable name="n" select="document('/db/doc.xml')//node[@id='2']"/>
is exactly as effective as:
<xu:grunt name="n" select="document('/db/doc.xml')//node[@id='2']"/>
or:
<xu:foo name="n" select="document('/db/doc.xml')//node[@id='2']"/>
or whatever... (by the way, is it normal for eXist not to report
unknown elements?)

I also tried to define the variables in the surrounding xquery, and
use them from within the xupdate, but I get an error saying "select
expression should evaluate to a node-set; got xs:integer". This is the
syntax I used:
let $node := document('/db/doc.xml')//node[@n='2']
let $parent := $node/..

let $mods := xmldb:update($root,
        <xu:modifications version="1.0" xmlns:xu="http://www.xmldb.org/xupdate";>
                <xu:append select="{$parent}" child="0">
                        <foo/>
                </xu:append>

        </xu:modifications>

)


Well, so here I'm stuck, is there any "official way" to replace nodes?


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