如何在XSLT转换中使用两个节点进行分组和计算值

<root>
  <item>
    <category>A</category>
    <value>10</value>
  </item>
  <item>
    <category>B</category>
    <value>20</value>
  </item>
  <item>
    <category>A</category>
    <value>30</value>
  </item>
  <item>
    <category>B</category>
    <value>40</value>
  </item>
</root>

<xsl:key name="group" match="item" use="category" />

<xsl:template match="root">
  <xsl:for-each select="item[generate-id() = generate-id(key('group', category)[1])]">
    <xsl:variable name="currentGroup" select="key('group', category)" />
    <xsl:value-of select="category" />
    <xsl:text>: </xsl:text>
    <xsl:value-of select="sum($currentGroup/value)" />
    <xsl:text>&#10;</xsl:text>
  </xsl:for-each>
</xsl:template>

A: 40
B: 60