<xsl:template match="TOC">
 <xsl:apply-templates select="STRUCT">
   <xsl:sort select="@pos" data-type="number"/>
   <xsl:sort select="STRUCT/@pos" data-type="number"/>
   <xsl:sort select="STRUCT/STRUCT/@pos" data-type="number"/>
   <xsl:sort select="STRUCT/STRUCT/STRUCT/@pos" data-type="number"/>
   ...
 </xsl:apply-templates>
</xsl:template>

HTH!
Anton

The depth of the STRUC elements is arbitrary.

If you know there is a resonable limit to the recusion depth you could do the following:

<xsl:sort select="number(concat(@pos, '.', STRUCT/@pos, STRUCT/STRUCT/@pos, STRUCT/STRUCT/STRUCT/@pos, ...)" data-type="number" />

Otherwise I would suggest a multistep process or refering to extensions..

HTH,
Geert