Just to further clarify, the transactions (and therefore transaction conflict detection) are more flexible than simple CAS operations. If transaction A reads key K1, and then writes to key K2, it will be rejected as conflicting if transaction B writes to key K1 and is serialized as happening before transaction K1 is committed (even though no other transaction wrote to key K2).
Every transaction in FoundationDB is submitted as a collection of writes/mutations, but also contains records of all keys read (and the consistent version at which they were read). In this test, the transactions all have empty conflict ranges, and thus cannot conflict with other transactions (but we still have to check!).
