  Modern blockchains face a scalability challenge due to the intrinsic
throughput limitations of consensus protocols. Layer 2 optimistic rollups (L2)
are a faster alternative that offer the same interface in terms of smart
contract development and user interaction. Optimistic rollups perform most
computations offchain and make light use of an underlying blockchain (L1) to
guarantee correct behavior, implementing a cheaper blockchain on a blockchain
solution. With optimistic rollups, a sequencer calculates offchain batches of
L2 transactions and commits batches (compressed or hashed) to the L1
blockchain. The use of hashes requires a data service to translate hashes into
their corresponding batches. Current L2 implementations consist of a
centralized sequencer (central authority) and an optional data availability
committee (DAC).
  In this paper, we propose a decentralized L2 optimistic rollup based on
Setchain, a decentralized Byzantine-tolerant implementation of sets. The main
contribution is a fully decentralized "arranger" where arrangers are a formal
definition combining sequencers and DACs. We prove our implementation correct
and show empirical evidence that our solution scales. A final contribution is a
system of incentives (payments) for servers that implement the sequencer and
data availability committee protocols correctly, and a fraud-proof mechanism to
detect violations of the protocol.
