Using XSL parameters to construct variables to constru

(Please forgive me, as I've been away from XSL for a while, and am struggling with a new project.) I'm transforming an XML file via JavaScript, based on user input. 

Approach 1: I'm currently creating a processor via JavaScript, then sending different parameters/modes to it to get search results. If the user chooses to narrow the search, he/she selects an option and I send a parameter to the stylesheet (processor). What I'd like to do inside the stylesheet is to construct a variable on the fly, based on the parameter values, then use that variable as the expression in the apply-templates select attribute value. Something like this:

XML snippet: 
<process>
     <number>1</number>
     <name>evaluate</name>
     <descrip>[Description.]</descrip>
</process>

XSL snippet:
<!-- default value of params is empty string; if user chooses a processArea option, I'll send the parameter to the stylesheet and set this value -->
<xsl:param name="processArea" /><!-- if sent, value will be 1, 2, 3 or 4 -->
. . . 
. . .
<xsl:variable name="selectStatement">/process-report/process
     <xsl:if test="$processArea != ''">[starts-with(number,<xsl:value-of select="$processArea"/>)]</xsl:if>
</xsl:variable>
. . . 
. . . 
<xsl:apply-templates select="$selectStatement"/>
. . . 
. . .

If a parameter is passed in, the parameter would not equal an empty string, so the corresponding predicate expression would become part of the variable. So I could add more predicates depending on the values of other parameters, then use the complete variable in the following apply-templates select statement.

This doesn't seem to work, because the variable will evaluate as a string instead of an expression, right? Is there a way around this?

Approach 2: I could also change the select attribute via script in the Web page, then select the node (using selectSingleNode) and change its value. Just wondering what the trade-offs would be between these two approaches (if the first approach is even possible, that is). Would it work faster, more efficiently?

Thanks,
Doug Howell

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