DOCBOOK: defining single quote element

Hi all,

I added the following to my custom html stylesheet:

----------------------------------------------

!DOCTYPE style-sheet PUBLIC "-//James Clark//DTD DSSSL Style Sheet//EN"
[
<!ENTITY cygnus-both.dsl SYSTEM
"/usr/lib/sgml/stylesheets/cygnus-both.dsl" CDATA DSSSL >
]>

<style-sheet> 
<style-specification id="html" use="cygnus-both">
<style-specification-body> 

;; HTML DocBook stylesheet personnalization

;; definition of a new element for (german) single quotes
(element quotesingle
    (make element gi: "SPAN"
          attributes: '(("CLASS" "QUOTESINGLE"))
              (make sequence
                (literal (gentext-start-nested-quote))
                (process-children)
                (literal (gentext-end-nested-quote)))))

[...........]

------------------------------------------------

Other definitions work (for ex.: (define %stylesheet% "manual.css") )

but my singlequote seems to be ignored:

$ /usr/bin/jade -t sgml -i html -d manual-html.dsl manual.sgml
/usr/bin/jade:manual.sgml:72:62:E: element "QUOTESINGLE" undefined  

Thanks for your help,

Camille.