XPath (SOAP UI): check if a specific namespace URI is defined - xpath

Question

I'm trying to write an assertion for a test step in SOAP UI using an XPath Match assertion. I want to check if a specific namespace URI is defined in the a REST service's response. I'm trying to use a combination of in-scope-prefixes and namespace-uri-for-prefix functions, but I can't make it work, so maybe somebody has an advice.
in-scope-prefixes(/other:element)
gives me a collection containing all defined prexies, I cant output it with
string-join(in-scope-prefixes(/other:element), ',')
So in a frist step I want to want to select the prefix of the namespace URI I'm looking for with
in-scope-prefixes(/other:element)[namespace-uri-for-prefix(.,/my:rootElement)='http://namespaceuri.org/my']
But SOAP UI gives me the error
Invalid XPath expression
Now I wonder if that expression is really invalid? Something like
in-scope-prefixes(/other:element)[concat(.,'-') = 'my-']
works without raising an error.Am I missing something?
EDIT1
I just realised that I'm able to achieve what I want using XQuery with
declare namespace other='http://namespaceuri.org/other';
let $rt := /other:rootElement
let $ret := exists(in-scope-prefixes($rt)[namespace-uri-for-prefix(.,$rt)='http://namespaceuri.org/my'])
return $ret
But this will not work:
declare namespace other='http://namespaceuri.org/other';
let $ret := exists(in-scope-prefixes(/other:rootElement)[namespace-uri-for-prefix(.,/other:rootElement)='http://namespaceuri.org/my'])
return $ret
For the latter, SOAP UI gives the exception
Invalid XQuery expression
It seems the behaviour is identical to XPath, except that I can use variables in XQuery.Does that make sense to anyone? Any idea how to get that working with XPath?