create multiparts message by xsl

i create multiparts message with xsl below but how can i give a content id to those parts?

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet

      version="2.0"

      xmlns:xsl="http://www.w3.org/1999/XSL/Transform"

      xmlns:XQMessageElem="http://www.sonicsw.com/sonicxq/com.sonicsw.xq.service.xform.TransformationElementFactory"

      extension-element-prefixes="XQMessageElem">

    <xsl:template match="/feed">

    <XQMessageElem:XQMessage>      

          <XQMessage>

                     <part>

                         <content><xsl:copy-of select="xsd"></xsl:copy-of></content>

                     </part>

                     <part>

                         <content><xsl:copy-of select="xsl></xsl:copy-of></content>

                     </part>                         

        </XQMessage>

    </XQMessageElem:XQMessage>

    </xsl:template>

</xsl:stylesheet>