Sequence of several values as the second argument of key() function

      <xsl:stylesheet
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>
            <xsl:key  name="kkk"
                        match="bbb"
                        use="@y"/>

            <xsl:template  match="/aaa">
                  <xsl:apply-templates  select="key('kkk',ccc/ddd)"/>
            </xsl:template>

            <xsl:template  match="*">
                  <xsl:copy-of  select="."/>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>
            <bbb  y="a">X</bbb>
            <bbb  y="b">Y</bbb>
            <bbb  y="c">Z</bbb>
            <ccc>
                  <ddd>a</ddd>
                  <ddd>c</ddd>
            </ccc>
      </aaa>

      <bbb  y="a">X</bbb>
      <bbb  y="c">Z</bbb>