Suppose I have two users who are issued two different [UProve](http://en.wikipedia.org/wiki/U-Prove) IDs. The Issuer has guaranteed that one UProve token bearer will never have more than one UProve token ID.

* How can I use UProve to guarantee that two users are "different" without disclosing their actual private ID?

Here is a scenario: An online voting system wants to use UProve to anonymously collect votes. They decide to trust an `Issuer` who has full knowledge of the end user, but the `relying pary` has to accept the fact that they will know nothing about the end user at all.

The polling station (`relying party`) only needs to know that the same UProve ID wasn't used to cast a second vote. The only UProve scenario I know of that would work here is *proving* the user's SSN (or similar fixed ID). 

In this case I don't want to disclose anything.

Is this possible?