PriorValue

What an output Block needs to undo a turn's writes to one key: its value before the turn first wrote it, encoded with the key's codec (None: no stored entry), and the fingerprint of what the turn's own writes last left there (written; None: no stored entry). A Block restores the key only while it still holds written, so a later write by anyone else - updateState while the turn waits for review - is kept (design §4.22).