Very Hard XSLT Transformation

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Hi XML Experts;

This is the Hardest Transformation I have ever had to do. Could someone help ??

I have an xml tree consisting of users
<root>
<user>
<firstName/>
<middleName/>
<lastName/>
</user>
<user>
<firstName/>
<middleName/>
<lastName/>
</user>
</root>

In my Transformation, I'm looping through user and sorting on lastName

But at the bottom of the resulting list I need to output the first unique letter of each last name ?

Example, If we have last names

Abe, Jimmy
Adams, Harry
Baldwin, Bob
Clinton, George
Fiona, Carly
Gates, Bill
Guntars, Jay
Williams, Wolly
Wilimina, Winston

Then I need to produce the following links below
A B C F G W
not
AABCFGGWW

<xsl:for-each select = &quot;root/user&quot; ><xsl:sort select=&quot;firstName&quot;/>
<xsl:variable name=&quot;currentLetter&quot; select = &quot;substring(firstName,1,1)&quot;/>
<xsl:value-of select=&quot;$currentLetter&quot;/>
<!--
<xsl:if test =&quot;$currentLetter!=$lastLetter&quot;>

<xsl:variable name=&quot;lastLetter&quot; select = &quot;substring(LName,1,1)&quot;/>
<xsl:value-of select=&quot;$lastLetter&quot;/>

</xsl:if>
-->
</xsl:for-each>

but you can't replace variable values in XSL ?
How would you guys approach this problem ? ?

Thanks
STev