[xsl] dynamic xpath question

[Duane Gran <duane.gran@xxxxxxxxx>]

I'm using XUpdate in an application where the XPath to the node I'm  
modifying is stored in the document.  For example, my document  
fragment looks like the following:

<form>
    <xpath>/mods/titleInfo/subTitle</xpath>
    <document>
        <mods>
            <titleInfo>
                <title>Sound and fury</title>
                <subTitle>my sub title</subTitle>
            </titleInfo>
         </mods>
    </document>
</form>

I'm trying to create an XUpdate variable which uses the data found  
in /form/xpath.  I'm trying the following without success:

<xu:variable name="path-to-node" select="/form/xpath"/>
<xu:variable name="sub" select="concat('/form/document', $path-to- 
node)"/>

Any advice on this would be most appreciated.  Thank you.

Duane Gran