Using a variable in XSLT

This may be an evergreeen question.
I would like to define a variable such that I can parametrize a selection.
The obvious would be:

<xsl:variable name="SystemSelected" select='"sys:one"' />
<xsl:template match="xs:documentation[lang('de')]/$SystemSelected">

However xsl:variable is a node set and not a string that can be used to 
concatenate within a match.
How can I handle this situation?
Regards to the helpful community.
TMS



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