Even if I attempt to use my dreaded *Devil's Advocate* technique, the two tests above effectively force me to provide the correct implementation. However, the cost is that the verification in `AcceptCallsVisitOnceWithCorrectType` is effectively a Strict Mock (as described in [a new blog post I wrote to explain the problem](http://blog.ploeh.dk/2013/10/23/mocks-for-commands-stubs-for-queries).

Is there any reason for not using [Data Flow Verification](http://blog.ploeh.dk/2013/10/23/mocks-for-commands-stubs-for-queries) instead?