Get all unique prefixes from a XML document

Hello,

I’ve created a XML document, but it isn’t complete, because I’ll still need to add the namespace declarations on top when the document is ready. Therefore I’d like to use XPath to get all prefixes, and based on that prefixes, add the namespaces. It’s not 100% correct to use fixed prefixes, I know, but it’ll solve my problem. I’ve been trying to do something with namespace-uri() finding the unique ones, but it didn’t work out. Can someone help me creating the right query?

//*[     not(  namespace-uri() = namespace-uri(preceding-sibling::node())  )     ]

Thanks in advance,

Robert de Wilde