Using a variable as an attribute name

One of the developers here is trying to do a test on a attribute where the
attribute name is contained in a variable.

We're getting an illegal character error on the test line.  Is what he's
trying to do even legal or are we taking the wrong tack on this?

  <xsl:variable name="attributeName" select="'charstyname'"/>
  <xsl:variable name="before" select="preceding-sibling::*[1]"/>
  
  <xsl:when test="not( $before[(_at_)$attributeName='Footnote'] )">
    [...]
  </xsl:when>

Thanks

        Dave

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