I'm having a think about this. I can see how it is OK for a pair of replicas, I need to think it through for a larger number. Maybe you're right to be concerned. The alternative of course is to have both dots…I'll get back to you. I think I can see the problem.

A, B and C. A and B concurrently add 'bob', so in A bob->{a,1} and in B bob->{b, 1}.
B merges with C so in C bob->{b, 1}
B merges with A and we pick B's dot bob->{b, 1}
C removes bob
C merges with A and since C's clock must have an entry for {b, 1} 'bob' is out of the set. Yet if the entry was {a, 1} bob would be in.

This suggests we need more than the dot.