path quesion

I am processing 2 XML files within a single xsl file. From one file
I retreive a path value e.g. 'customer/age', from an element , this value
is placed in a variable. I then want to look for the 'customer/age'
element in the 2nd xml file, e.g.


	variable $var contain the sting 'customer/age'

     attempt to use this in a path expression for the second document

    <xsl:value-of select="document('data.xml')/$var" />

I get the following error

   org.apache.xalan.xslt.XSLProcessorException: pattern =
'document('data.xml')/data/$id'


thankful for any help


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