>  xmlns:mml="http://www.w3.org/1998/Math/MathML"
ah, my prefix:-)



> but unfortunately when I transform my sample xml file
> with your xslt (+ the stylesheet definition above)
> Xalan-J complains that the namespace prefix mml is not
> declared. Am I still missing something ?

You showed a correct declaration of the namespace prefix and then said 
xalan gave you an error that it wasn't declared. So presumably xalan was
looking at a different file. Perhaps you are using xsl:include, the
prefix needs to be declared in every file (stylesheet or input file) in
which it is used.

David

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