Grouping with second choice

I want to group students according to their online registration which is

exported to xml similar to the file attached below (students.xml).
Registration
allows students to express a first and a second choice for their
exercise. Using
the Muenchian Method is appropriate for sorting with respect to a single

key, but how do I realize grouping with constraints, e.g. use the second

choice if the first cannot be fulfilled (group is full)? Any hints?

Thorsten

----------------------- classification.xsl
--------------------------------
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:output method="xml"/>

<xsl:key name="students-by-firstchoice" match="Student"
use="FirstChoice"/>

<xsl:template match="Course">
  <xsl:element name="Classification">
 <xsl:for-each select="Student[count(. | key('students-by-firstchoice',
FirstChoice)[1]) = 1]">
   <xsl:sort select="FirstChoice" />
         <xsl:element name="{FirstChoice}">
          <xsl:for-each select="key('students-by-firstchoice',
FirstChoice)">
      <xsl:sort select="RegistrationDate" />
              <xsl:if test="position() &lt;= 3">
           <xsl:copy-of select="."/>
        </xsl:if>
        <xsl:if test="position() &gt; 3">
           <!-- use second choice for grouping -->
        </xsl:if>
          </xsl:for-each>
        </xsl:element>
      </xsl:for-each>
   </xsl:element>
</xsl:template>

</xsl:stylesheet>

----------------------- students.xml -------------------------------
<?xml version="1.0" encoding="ISO-8859-1" ?>
<Course>
   <Student>
      <RegistrationDate>20010725</RegistrationDate>
      <FirstChoice>Group1</FirstChoice>
      <SecondChoice>Group2</SecondChoice>
      <Name>B</Name>
   </Student>
   <Student>
      <RegistrationDate>20010724</RegistrationDate>
      <FirstChoice>Group1</FirstChoice>
      <SecondChoice>Group2</SecondChoice>
      <Name>A</Name>
   </Student>
   <Student>
      <RegistrationDate>20010726</RegistrationDate>
      <FirstChoice>Group1</FirstChoice>
      <SecondChoice>Group2</SecondChoice>
      <Name>C</Name>
   </Student>
   <Student>
      <RegistrationDate>20010723</RegistrationDate>
      <FirstChoice>Group2</FirstChoice>
      <SecondChoice>Group1</SecondChoice>
      <Name>E</Name>
   </Student>
   <Student>
      <RegistrationDate>20010722</RegistrationDate>
      <FirstChoice>Group2</FirstChoice>
      <SecondChoice>Group1</SecondChoice>
      <Name>D</Name>
   </Student>
   <Student>
      <RegistrationDate>20010727</RegistrationDate>
      <FirstChoice>Group1</FirstChoice>
      <SecondChoice>Group2</SecondChoice>
      <Name>F</Name>
   </Student>
</Course>


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