how to refer to original source tree?

Given that one can refer to an alternate source document with the document()
function, how does one go about referring to the original source tree when
not in that context? For example, pretend I want to compare the value of a
'foo' element in the alternate tree with a value in the original tree like
this:

...
<xsl:variable name="More_Nodes"
select="document('another_source_tree.xml')"/>
<xsl:apply-templates select="$More_Nodes" mode="Not_In_Original_Tree"/>
...

<!-- process 'foo' elements in the $More_Nodes node-set -->
<xsl:template match="foo" mode="Not_In_Original_Tree">
  <!-- how would I do this? -->
  <xsl:value-of select="/path/to/something/in/original/tree = text()"/>
</xsl:template>

   - Mike
________________________________________________________________________
 Mike Brown / Hyperreal   |  Hyperreal http://www.hyperreal.org/music/
 PO Box 61334             |     XML & XSL http://www.skew.org/xml/
 Denver CO 80206-8334 USA |       http://www.hyperreal.org/~mike/


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