Using variables in xsl:sort select attribute

I have defined a variable called $sortby, and later in the template, wish to
display a list of elements and sort the list according to the value stored
in $sortby

...

<xsl:template match="customerList">
<xsl:variable name="sortby" select="@sortby">

<xsl:for-each select="person">
<xsl:sort select="$sortby"/>
	<tr>
	<td><xsl:value-of select="name" /></td>
	<td><xsl:value-of select="age" /></td>
	</tr>
</xsl:for-each>

</xsl:template>

...


However, this doesn't work. The parser ignores the sort instruction. Am I
permitted to use a variable as the argument for the select attribute of the
sort?

Mike




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