Recently I had a need for something similar to "xUnit contract tests" in Fitnesse (defined as: a test suite, which any object must satisfy so that it can be considered as a valid implementation of a Role. I got the term from one of J.B.Rainsberger's talks. )
To illustrate my point, let me cook up an example.
Imagine a contest for BoneyM impersonators. We would quiz each impersonator with a set of questions that they must answer correctly to win the contest.
i.e. we have a set of tests (questions) for each implementation of an Impersonator (Role). We also have multiple impersonators that I want to validate against the contract tests. The tests need to be dry - since they don't change for each implementation.