XSL样式表排序有关问题

XSL样式表排序问题
请大家帮忙看下这个问题怎么解决

XML code

XML文件:...<aa><a num="2" /><a num="3" /><a num="21" /><a num="1" /></aa>

XML code

XSL样式表文件:...<div><xsl:for-each select="aa/a"><xsl:sort select="@num" /><span><xsl:value-of select="@num" />,</span></xsl:for-each></div>...

XML code

<div><xsl:for-each select="aa/a"><xsl:sort select="@num"　data-type="number" /><span><xsl:value-of select="@num" />,</span></xsl:for-each></div>