如何使用XSLT将字符串转换为大写或小写？

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:variable name="inputString" select="'Hello, World!'" />
    <xsl:variable name="upperCaseString">
      <xsl:value-of select="translate($inputString, 'abcdefghijklmnopqrstuvwxyz', 'ABCDEFGHIJKLMNOPQRSTUVWXYZ')" />
    </xsl:variable>
    <xsl:variable name="lowerCaseString">
      <xsl:value-of select="translate($inputString, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz')" />
    </xsl:variable>
   <output>
     <upperCase>
        <xsl:value-of select="$upperCaseString" />
      </upperCase>
     <lowerCase>
        <xsl:value-of select="$lowerCaseString" />
      </lowerCase>
    </output>
  </xsl:template>
</xsl:stylesheet>