I have a feeling that my original code for UPC was right, as seen also
in the "Check digit calculation" section here,
http://en.wikipedia.org/wiki/Universal_Product_Code.

Inspired by Abel, I have now made an improved version of my original
code for testing if UPC code is legal. The two versions, both tested,
are shown side by side.

<?xml version="1.0"?>
<!-- legal UPC e.g.: 639382000393-->
<test>
   <upc>639382000393</upc>
</test>

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0"
   xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs">
   <xsl:output indent="yes"/>
   <xsl:strip-space elements="*"/>
   <xsl:template match="test/upc">
       <topelement>
           <title>Test of UPC code. Legal code returns empty elements.</title>

<original>

               <xsl:variable name="x"
select="((xs:integer(substring(., 1, 1)) + xs:integer(substring(.,
3,1)) + xs:integer(substring(., 5, 1)) + xs:integer(substring(., 7,
1)) + xs:integer(substring(., 9, 1)) + xs:integer(substring(., 11,
1))) * 3 + (xs:integer(substring(., 2, 1)) + xs:integer(substring(.,
4, 1)) + xs:integer(substring(., 6, 1)) + xs:integer(substring(., 8,
1)) + xs:integer(substring(., 10, 1)))) mod 10"/>

               <xsl:if test="(if ($x ne 0) then (10 - $x) else $x) ne
xs:integer(substring(., 12, 1))">UPC not legal</xsl:if>

</original>

<improved>

               <xsl:variable name="y" select="for $i in
string-to-codepoints(substring(., 1, string-length(.) - 1)) return
codepoints-to-string($i)"/>

<xsl:variable name="z" select="sum((for $i in $y[(position()) mod 2 =
1] return xs:integer($i)*3, for $i in $y[(position()) mod 2 = 0]
return xs:integer($i))) mod 10"/>

               <xsl:if test="(if ($z ne 0) then (10 - $z) else $z) ne
xs:integer(substring(., string-length(.), 1))">UPC not legal</xsl:if>

           </improved>
       </topelement>
   </xsl:template>
</xsl:stylesheet>

Running that gave me 7 - when the check digit should be 3 shouldn't
it?  You don't appear to be doing the mod 10 calculations correctly
(according to the steps on wikipedia) - you do the summing, mod 10 it,
take that from 10 and then mod 10 it again.

 <xsl:variable name="tokens"
   select="for $i in 1 to string-length(.) return
xs:integer(substring(., $i, 1))"
   as="xs:integer+"/>

 <xsl:variable name="code" select="$tokens[position() ne last()]"
as="xs:integer+"/>
 <xsl:variable name="check-digit" select="$tokens[last()]" as="xs:integer+"/>
 <xsl:variable name="odd" select="$code[position() mod 2 = 1]"
as="xs:integer+"/>
 <xsl:variable name="even" select="$code[position() mod 2 = 0]"
as="xs:integer+"/>

 <xsl:variable name="calc" select="(10 - ((sum($odd) * 3) +
sum($even)) mod 10) mod 10" as="xs:integer"/>

 <improved tokens="{$tokens}" code="{$code}"
check-digit="{$check-digit}" calc="{$calc}">
   <xsl:if test="$calc ne $check-digit">UPC not legal</xsl:if>
 </improved>

<test>
 <upc>639382000393</upc>
 <upc>036000291452</upc>
</test>

<improved tokens="6 3 9 3 8 2 0 0 0 3 9 3" code="6 3 9 3 8 2 0 0 0 3
9" check-digit="3"
            calc="3"/>
<improved tokens="0 3 6 0 0 0 2 9 1 4 5 2" code="0 3 6 0 0 0 2 9 1 4
5" check-digit="2"
            calc="2"/>

cheers
andrew