ABD write-back needs replicas to ack (not reject) same-version writes

When describing or implementing ABD (multi-writer atomic register), a common mistake is saying replicas 'reject writes with older or equal versions'. The get's write-back phase usually carries the version the replicas already hold, so a replica must keep its copy but still return OK for an equal or older version; if it rejected, the write-back could never reach a quorum and reads would time out. Only a same-version-different-value write is a genuine conflict worth erroring on.