My code for handling <biblStruct> has been working fine for a number of
years, but I just recently tried to add the sort by <surname>. The
relevant part of the code looks like this (including my most recent
attempt at sorting, which generates error messages):

  <!-- Bibliographical renderings -->
<xsl:template match="TEI//listBibl">
  <xsl:for-each select="biblStruct">
    <xsl:sort select="*//surname"/>
   <p style="text-indent:-10mm;margin-left:10mm">
    <xsl:if test="@n">
     <span style="font-weight:bold;margin-left:2mm">
      <xsl:value-of select="@n"/>
      <xsl:text>-&#x09;</xsl:text>
     </span>
    </xsl:if>
      <xsl:apply-templates/>
   </p>
  </xsl:for-each>
</xsl:template>