XSL/XPath Dynamic sort key

<xsl:param name="sort_select_lvl_1">author</xsl:param>
<xsl:param name="sort_select_lvl_2">/</xsl:param>
<xsl:param name="sort_select_lvl_3">last_name</xsl:param>
<xsl:param name="sort_order">ascending</xsl:param>
<xsl:param name="sort_data_type">text</xsl:param>

xsl:sort select="concat('author','/','last_name')" />
is not the same as
<xsl:sort select="author/last_name" />

<catalog>
 <document>
   <uid>2301</uid>
   <url>http://www.who.com/document.htm</url>
   <subject>General Stuff</subject>
   <abstract>This is an abstractabstract>
   <accesses>1</accesses>
   <age>4</age>
   <last_accessed date="20011229">12/29/2001</last_accessed>
   <author>
     <first_name>Peon</first_name>
     <last_name>lover</last_name>
     <serial>9A1551645</serial>
   </author>
   <owner>
     <first_name>Jolly</first_name>
     <last_name>Goose</last_name>
     <serial>123456</serial>
   </owner>
   <manager>
     <first_name>Someone</first_name>
     <last_name>Special</last_name>
     <serial>123456</serial>
   </manager>
   <supervisor>
     <first_name>Someone</first_name>
     <last_name>Important</last_name>
     <serial>123456</serial>
   </supervisor>
 </document>
 ...
</catalog>