how to changing context

I have a XSLT page to transform a.xml as well as an XML
file referenced by document('b.xml')

in the XSLT program, i wrote:

<xsl:template match="/"> <!-- the root of a.xml -->

  <xsl:for-each select="document('b.xml')/root/subroot"/>
       <!-- here I want to reference to both
            the nodes under EACH "subroot" of b.xml
            and the nodes that are under the "/"node of a.xml>
  </xsl:for-each>

</xsl:template>

how can i change context inside <xsl:for-each>? (better without the help
of variable/param)

Thanks a lot!



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