adding or modifing an attribute

[Sam Carleton]

I have worked with DOM, SAX, XSLT, and XPath in the past and am now
working with a framework that has XQuery.  The task at hand is very
simple, change (or add if not present) an attribute to one element in
an XML document:

Example: If the name element has an average attribute, change it to
93, otherwise create it and set it to 93
input:

<root>
 <students>
   <name value="scott"/>
 </students>
</root>

or

<root>
 <students>
   <name value="scott" average="90"/>
 </students>
</root>

output:

<root>
 <students>
   <name value="scott" average="93"/>
 </students>
</root>

How might I do this with XQuery?  The framework I am using is
Trolltech's Qt which is a C++ framework.  I am under the impression I
am going to have to load the XML document into memory, execute the
XQuery query on it that will result in a new XML document that I will
then save over the original XML file, is that correct?  I can figure
out the Qt stuff, the real question is how would I put together a
query to make this change in XQuery?

Sam