DOCBOOK: Best way to have an XML-document which can be parsed as SGML?

[Thorbjoern Ravn Andersen <ravn@mip.sdu.dk>]

I would like to have my DocBook XML documents in a form in which I can
equally well apply XSL-stylesheets with Cocoon as process it with Jade (to
get RTF and PDF)

Still being a newbie, I would appreciate a hint regarding how to do this
as "properly" as possible. 

Can I have a common header for the two processing types (where the SGML
tools ignore the <?xml and <?xsl tags) or should I create a seperate
header which is prepended when I run Jade?

Pointers and thoughts appreciated :-)

-- 
  Thorbjørn Ravn Andersen               "...plus...Tubular Bells!"
  http://www.mip.sdu.dk/~ravn