按Xsl中的子级值对xml节点进行分组

<root>
<element>
<id>1</id>
<group>first</group>
</element>

<element>
<id>2</id>
<group>second</group>
</element>


<element>
<id>3</id>
<group>first</group>
</element>
...
<root>
<root>
<element>
<id>1</id>
<group>first</group