Adding variables to the definitions tag

Maybe this is a FAO, but I'll give it a try anyway...

I'm using the <xsl:attribute> to add new namspaces to my definitions tag. It
works fine when I specify the namspace like this:
<xsl:attribute name="xmlns:bla">"http://www.bla.org/bla"
</xsl:attribute>		

But I want to make it more flexible and use different namespace values from
different xml files. So I get the namespace values and assign them to
variables, which is no problem. But I cannot use <xsl:attribute> for adding
to the definitions tag, because I'm not allowed to use variable names inside
the <xsl:attribute> tag. 
So my question is, does anybody know what I should use for adding them my
definitions tag? 



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