><!-- Expected Output -->
>xml journal
>xsl journal
>css journal

Hi Kurt,

Instant Saxon 6.0.2/6.1/6.2 and Xalan 1.2.2. seem to be on better terms
with preceding-sibling:: then with preceding:: 

<xsl:variable name='allpubs' 
select="//pubtag[@list='yes']/@pub[not(. =
../preceding-sibling::pubtag/@pub)]" />

produces your expected output. For a general solution, though,  Jeni's
  /publist/pubtag[@list = 'yes']
                 [count(.|key('pubs', @pub)[1]) = 1]
is sound advice (as always).

Cheers,
Eric

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list