Happily, this seems to sort numerically rather than lexicographically,
although as far as I can tell I've not put anything in the file to
indicate that that's what I wanted.
<z:index name="zeerex:reliability" type="n">
<xsl:value-of select="e:serverInfo/e:reliability"/>
</z:index>
+ <z:index name="zeerex:reliability" type="s">
+ <xsl:value-of select="e:serverInfo/e:reliability"/>
+ </z:index>
<z:index name="dc:date" type="d">
<xsl:value-of select="e:serverInfo/e:database/@lastUpdate"/>
</z:index>