[xsl] Can group-by treat its target it two ways?

Subject: [xsl] Can group-by treat its target it two ways?
From: "Mark" <mark@xxxxxxxxxxxx>
Date: Sat, 5 Nov 2011 10:38:06 -0700
I have sorted a file into Czech collation order (Thanks Michael) so that I have a series of <Word> elements that identify a unique word and all of its citations for various documents. I next want to combine all the words that start with the same letter into a file that has the same name as the initial letter of the word. For English words, this was easy:
<xsl:for-each-group select="Word" group-by="lower-case(substring(@word,1,1))">
<xsl:result-document href="{concat('index/en-', current-grouping-key())}.htm">

but Czech has a diagraph 'ch' that comes after 'h'. At the moment, I can produce the list of <Word> elements in the correct order ('ch' follows 'h') with
<xsl:for-each-group select="Word" group-by="@word">
<xsl:sort select="@word" lang="cs"/>
but its granularity is at the <Word> level. How can I make
<xsl:for-each-group select="Word" group-by="lower-case(substring(@word,1,1))"> consider 'ch' as a single letter? (Right now, it is tacking all of the 'ch' words at the end of the 'c' file because of the substring(@word, 1,1) filter.
Or can you see a better approach?

My sorted word list in Czech collated order:
<Word word="hrad">
   <Citation ....../>
   <Citation ...../>
<Word word="chod">
   <Citation ....../>
  <Citation ...../>

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