Optimistic cheap multi-exit for Plasma (Cash or MVP)

[danrobinson]

@vbuterin

We can make exiting from Plasma cheaper in the normal case by simplifying the contents of an exit transaction; instead of actually including a Merkle proof, a transaction would simply contain a list of (block number, coin index) pairs. There would be an extra type of challenge mechanism where anyone could challenge to require you to actually provide a Merkle branch in order to complete a withdrawal.

You may be able to even skip providing the block number… just the coin indexes.

I think this ends up being similar to a cryptoeconomic aggregate signature, where consent of the parties involved is assumed unless they challenge within a set time. So this idea is to “verbose” withdrawal as Plasma XT is to explicit withdrawal-and-redeposit (i.e. manual on-chain checkpointing).