A key-value store with sets of parties as keys, and with LtHash16 values. Keeps a watermark of the record time (a timestamp accompanied by a tie-breaker, to account for multiple changes with the same timestamp) of the last update.

While the store is agnostic to its use, we use is as follows. For each set S of parties such that:

• the parties are stakeholders on some contract C and
• the participant stores C in its ACS

the participant uses the store to store an LtHash16 commitment to all the contracts whose stakeholders are exactly S.

To ensure that the commitments correspond to the ACSs, the caller(s) must jointly ensure that all ACS changes are delivered to this store exactly once. In particular, upon crashes, the caller(s) must send ALL ACS changes that are later - in the lexicographic order of (timestamp, request) - than the watermark returned by the store, but must not replay any changes lower or equal to the watermark.