Optimistic concurrency countermeasure when writing to an event store: the write specifies the expected current version of the stream, and the store rejects the write if another writer has appended first. Important when two threads/requests concurrently issue commands against the same entity. Must not be applied blindly — after an irreversible external side effect (e.g. charging a credit card through Stripe) you do not want the write to fail, since that would cause double-charging on retry. Because different situations call for different write-time behaviour, the handler — not a generic framework writer — must control event writing.