Token-level streaming differ with rollback

Question

Design an incremental processor for a token stream whose producer may append new tokens or roll back previously emitted tokens.

The processor must always expose the latest sequence while doing work proportional to the changed suffix rather than rebuilding all derived state after every update. Define the update interface and the state needed to support append, rollback, and replacement.

Explain how you would handle duplicate updates, rollback beyond retained history, bounded memory, persistence, and consumers that are temporarily offline.

## Follow-ups

1. Attach versions so consumers can reject stale or duplicate updates.
2. Compact retained history without breaking a valid rollback window.
3. Recover after a restart while preserving exactly which suffix each consumer has seen.