The average user would correctly say that the text returned from the request. But that same average user would not balk at all with there being multiple representations of the same resource. By definition that is the case. I.e. you want the impossible. Plus, who really cares if x' and y' might be equal according to some third part.
URNs are URIs.
A URN is simply a URI scheme which has the required properties of non-reassignability and uniqueness. I.e. once a URN is bound to its resource, at any time in the future, you MUST assume that it is still bound to that resource.