How to define a xsl:sort's order attribute using a va

I want to sort a list of <person> elements, and define the "select" and
"order" attributes of the xsl:sort element using variables, rather than
hardcoded values.

xml source:

<personlist sortBy="name" sortOrder="descending">
 <person>
  <name>Bob</name>
  <age>40</age>
 </person>
 <person>
  <name>Mary</name>
  <age>53</age>
 </person>
 <person>
  <name>Arthur</name>
  <age>22</age>
 </person>
</personlist>


Here's an example of what I want to do. Iterate through the list of <person>
elements, and sort by the element whose names matches personlist/@sortBy, in
the order defined by personlist/@sortOrder:

<xsl:template match="personlist">

 <xsl:variable name="sortBy">
  <xsl:attribute name="value">
   <xsl:value-of select="@sortBy">
  </xsl:attribute>
 </xsl:variable>

 <xsl:variable name="sortOrder">
  <xsl:attribute name="value">
   <xsl:value-of select="@sortOrder">
  </xsl:attribute>
 </xsl:variable>

 <xsl:for-each select="person/*[name()=$sortBy]">
 <xsl:sort select="." order="$sortOrder"/>

  <p>
  <xsl:value-of select="../name"/>&#160;
  <xsl:value-of select="../age"/>
  </p>

 </xsl:for-each>

</xsl:template>


.. This does not work. I cannot define the value of the order attribute
using a variable. Is there any way I can do this? 

Mike

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