finding namespace-URI for a given prefix

hello,
my head gone in circles trying to get the following:

- given a stylesheet in which namespaces with prefixes are defined (may also be any other xml format but thats the one i am interested in)
	<xsl:stylesheet xmlns:n1="n1uri" xmlns:n2="n2uri" xmlns:xsl=[...]
		<xsl:namespace-alias stylesheet-prefix="n1" result-prefix="n2"/>
	[...]

- find for each namespaceprefix (e.g. for /xsl:stylesheet/namespace-alias/@stylesheet-prefix) the used URI...

i tried:
- xmlns:XXX is not an attribute so cannot be "xpathed" or matched with someling like @xmlns:*
- the namespace-uri() function does only give the default namespace for an element, so cannot find other declared and prefixed namespaces
- i also thought the dyn:evaluate extension function might help, but this still cannot work as i cannot use the namespace n1 in the xpath if it is not declared...

so i think this is not possible using xslt alone. do i oversee something?

thanks 
chris