Sorting and grouping

Hi,
can someone help me with the following problem?

I have an unordered list of items: 
<list>
    <item at="...">
        ...
    </item>
    ...
    <item at="...">
        ...
    </item>
</list>

I want to sort this list by an 'at' attribute and then display these
sorted items in groups. 
Each group should consist of a fixed number (let's say 5) of items. I
can solve this problem 
only by first sorting the list into a temporary .xml file and then use
this file for grouping.
What's even worse, I'am not able to handle the problem in one single xsl
transformation file.

To be more specific, one can't do it in one pass like this:
        <xsl:apply-templates select="item">
            <xsl:sort select="@at" order="descending"/>
        </xsl:apply-templates>
	....
        <xsl:template match="item[(position() mod 5) = 1]">
            <xsl:variable name="group"
select=".|following::item[position() &lt; 5]"/>
because last 'select' selects following items in document order (not in
sorted order).

Is there a way to solve that without creating a temporary file?

Thank you for your help.
 Zdenek

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