[xsl] string parameter to node-set?

["bernd@xxxxxxxxxxxxx" <bernd@xxxxxxxxxxxxx>]

I am at a loss, and wonder if anybody can help: I pass two parameter 
from Javascript into XSLT, the URI of an XML document and an XPath 
expression. Both parameters are strings as far as JavaScript is concerned.

I can load the XML data with document($para-xml) just fine:

   <xsl:param name="para-xml"/>
   <xsl:param name="para-xpath" select="'/'"/>

   <xsl:template match="/">
       <xsl:apply-templates select="document($para-xml)" mode="transform"/>
   </xsl:template>

Next, I would like to use the XPath expression to select the node(s) the 
subsequent transformation should apply to. Something along the lines of 
this:

   <xsl:template match="/" mode="transform">
       <xsl:apply-templates select="/root/data[@id='whatever']"/>
   </xsl:template>

That works great, too. However, I find myself unable to using the XPath 
parameter instead of the constant XPath expression, as in

   <xsl:template match="/" mode="transform">
       <xsl:apply-templates select="$para-xpath"/>
   </xsl:template>

I suppose this is because the parameter is of type string, and the 
select attribute requires a node set. How do I solve this?

The key problem is that I need to transform a subset of a larger XML 
stream, where the subset is defined through an XPath expression and all 
this must happen within XSL in order to avoid cross-domain security 
constrains in JavaScript. Any recommendations welcome.

TIA
Bernd


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