Sorting Question: Muenchian method? Substring in Key?

Hello. This is my first XSLT project. I'm having trouble with grouping
sorted items (seems to be a common theme). I have a XML file like this
(simplified):

<topics>
  <topic>Anteater</topic>
  <topic>Aardvark</topic>
  <topic>Beetle, Smaller European Elm Bark</topic>
  <topic>Beetle, American Burying</topic>
  <topic>Chimpanzee</topic>
</topics>

I would like the output to be sorted and grouped, with each group getting a
heading:

A
Aardvark
Anteater

B
Beetle, American Burying
Beetle, Smaller European Elm Bark

C
Chimpanzee

The corresponding part of my XSL is below.

  <xsl:template match="topics">
   <table border="1">
      <xsl:for-each select="topic">
        <xsl:sort select="."/>
        <xsl:variable name="previndex" select="position()-1"/>
        <xsl:if test="not(substring(.,1,1)
                      = substring(/topics/topic[$previndex],1,1))">
                <tr><th><xsl:value-of select="substring(.,1,1)"/></th></tr>

        </xsl:if>
            <tr>
              <td><xsl:value-of select="."/></td>
            </tr>
      </xsl:for-each>
    </table>
  </xsl:template> 

As you can see, I'm comparing an element in a sorted node-set with a
document-order node-set. This works only if the XML is pre-sorted. Is there
a way to create the sorted node-set first, so that I can both traverse and
use it in my test?

I have seen a few examples of the Muenchian method, and it would seem this
is another (perhaps better) way solve my problem. However, I've tried
several times to implement it and can't seem to get it right (can you use a
substring in a key?)

Thanks for any insight,

- Mark.

-- 
Mark Thomas                       |_|
Internet Systems Architect        -+-
User Technology Associates, Inc.   |
Thomas_M(_at_)bls(_dot_)gov                  /-\




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