sum() function

Hello,
i have a XML like:

<root>
<tag>
        <tagID>FA40</tagID>
        <tagF>
                <tagG>25</tagG>
        </tagF>
</tag>
<tag>
        <tagID>RR</tagID>
        <tagF>
                <tagG>50</tagG>
        </tagF>
</tag>
<tag>
        <tagID>FA40</tagID>
        <tagF>
                <tagG>25</tagG>
        </tagF>
</tag>
</root>

and I want to apply the function sum() but only when tagID = FA40.
Can somebody help me?
thanks in advance

david


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