From docbook tdg.
computeroutput: Inline markup.
screen. A block level element
This element is displayed “verbatim”; whitespace and linebreaks within
this element are significant.
userinput:
Note that UserInput is not a verbatim environment, but an inline.
Does that help explain some of the oddities?
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Regards DaveP.
XSLT&Docbook FAQ
http://www.dpawson.co.uk/xsl