grouping node-sets in an external document

I'm trying to apply the Muenchian grouping method to an external document.  Of 
course, this doesn't work because the document function doesn't work within 
match statements.  Am I going around this the wrong way?

newFirstGraders.xml
<class name="101" type="math">
   <student>Bob</student>
   <student>Joe</student>
   <student>Mary</student>
</class>
<class name="201" type="science">
   <student>Bob</student>
   <student>Joe</student>
   <student>Mary</student>
</class>
<class name="301" type="math">
   <student>John</student>
   <student>Peter</student>
</class>

firstGraders.xml
<class name="101" type="math">
   <student>Fred</student>
   <student>Mark</student>
</class>
<class name="201" type="science">
   <student>Paul</student>
</class>
<class name="301" type="math">
   <student>Wendy</student>
</class>

generateSchoolReport.xsl
<?xml version="1.0"?>
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; version="1.0">
<xsl:output method="xml" indent="yes" />

<xsl:key name="newstudents" match="document('class101.xml')/class/student" 
use="../@type" />
<xsl:key name="students" match="/class/student" use="../@type" />

<xsl:template match="/class/student">
   <xsl:apply-templates 
select="/class/student[generate-id(.)=generate-id(key('students', ../@type))]" 
/> mode="uniqueType" />
</xsl:template>

<xsl:template match="/class/student" mode="uniqueType">
Number of Math Students: <xsl:value-of select="count(key('students','math')) + 
count(key('newstudents', ../@type))" /></xsl:attribute>
</xsl:template>

</xsl:transform>

Is there a way to do this (while still grouping unique groups across 
documents)? 
 Has this already been covered.  It feels familiar, but I couldn't find a 
relevant solution through searching.  I know this XSL looks bad -- I'm new... 
and this XSL was extracted from a far more complex XSL algorithm.  Given this 
example, I'm sure you can write a better template match that does not require 
the 'function' call trick of using a mode='uniqueType'.  I'm just too lazy to 
make a better example, sorry.

Carl


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