The position() function in xsl:for-each-group

      <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:template  match="/aaa">
                  <xsl:for-each-group  select="*"
                              group-by="@v,@q">
                        <group-members  id="{current-grouping-key()}"
                                    pos="{position()}">
                              <xsl:apply-templates  select="current-group()"/>
                        </group-members>
                  </xsl:for-each-group>
            </xsl:template>

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

      </xsl:stylesheet>

      <aaa>
            <bbb  v="v2"
                        q="q0"/>
            <ccc  v="v1"
                        q="q0"/>
            <ddd  v="v1"/>
            <eee  v="v2"/>
            <fff  v="v1"
                        q="q1"/>
            <ggg  v="v3"
                        q="q1"/>
      </aaa>

      <group-members  id="v2"
                  pos="1">
            <bbb  v="v2"
                        q="q0"/>
            <eee  v="v2"/>
      </group-members>
      <group-members  id="q0"
                  pos="2">
            <bbb  v="v2"
                        q="q0"/>
            <ccc  v="v1"
                        q="q0"/>
      </group-members>
      <group-members  id="v1"
                  pos="3">
            <ccc  v="v1"
                        q="q0"/>
            <ddd  v="v1"/>
            <fff  v="v1"
                        q="q1"/>
      </group-members>
      <group-members  id="q1"
                  pos="4">
            <fff  v="v1"
                        q="q1"/>
            <ggg  v="v3"
                        q="q1"/>
      </group-members>
      <group-members  id="v3"
                  pos="5">
            <ggg  v="v3"
                        q="q1"/>
      </group-members>