XPaths made up of values of nodes specified by other XPaths

Has anyone come up with a way of using an XPath, part of which is found by
taking the value of a node specified by another XPath? In particular, using
an extension function rather than variable assignment.

Say I have the following document:

<A>
    <B C="E/F"/>
    <D>
        <E>
            <F>JACKPOT!</F>
        </E>
    </D>
</A>

and I want to reference the F as /A/D/[whatever the value of /A/B/@C is]

I'd like to be able to do something like:

    /A/D/foo("/A/B/@C")

Is such an extension function feasible?

James


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