noted that verified timestamps have more use-cases

A believer can be convinced that a sigsum was logged after time T.  This
is because witnesses do Verifier(append-only) and Verifier(Freshness).

Outline: a claimant is about to log a sigsum.

1. Fetch the most recent cosigned tree head.
    - Timestamp is T
    - Tree size is N
2. Submit sigsum for logging.
3. Wait for inclusion at index N+k, k=>0.
4. Wait for next cosigned tree head.
    - Timestamp is T', where T' > T
    - Tree size is N', where N' > N+k
5. Download inclusion proof for tree size N'.

Now you can convince a believer that a sigsum is publicly logged.  Just
reveal inclusion proof which leads up to the second cosigned tree head.

Next, you can reveal the first cosigned tree head that _have not merged
that entry yet_.  This follows from the first cosigned tree head size,
and makes it obvious that the entry must have been merge after time T.

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