1.) Rollups opt-in for registration through sidecar, which forwards the information to be stored in SEQ. This ensures their inclusion in the auction at epoch n on Arcadia.
2.) getBuilder() returns builder for epoch n.
3.) GetPayload() allows Sidecar to fetch the rollups' blocks once it has been preconf'd on Arcadia. Sidecar provides the information needed to Arcadia for a response. Rollup then checks the public key of the current builder vs the builder who signed the rollup block to ensure they got a valid block.
4.) Rollup wants to deregister(opt-out) of Arcadia on epoch n+k, so it should send opting out transaction by end of epoch n+k-2.