using variable's value in select statement

I am using a stylesheet to read a string from an xml file into a
variable.  Now I want to use that string in a select xsl:value-of
statement.

XML file 1:

	<?xml version="1.0"?>
	<Data>
		<value1>1</value1>
		<value2>3</value2>
		<sum>4</sum>
	</Data>

XML File 2:

	<?xml version="1.0"?>
	<Functions>
		<Func1>value1 + value2</Func1>
	</Functions>

**********
In the XSL:

	<xsl:variable name="thisline"><xsl:value-of
select="$funcfile/Functions/Func1" /></xsl:variable>    

This returns a string which is actually a mathematical function acting
on other xml elements

Now I want to take this string(function) and use xsl:value-of to get the
value of the fuction within the xml
so it can be used in other places


	<xsl:variable name="thisfunc"><xsl:value-of select="$thisline"
/></xsl:variable>
	<xsl:if test="$thisfunc = sum">
		<xsl:text>The sum is correct</xsl:text>
	</xsl:text>

Hope this makes sense!

Thanks


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