https://zmnscpxj.github.io/bitcoin/unchained.html

Seems on point.

Basically - a federated group runs the onchain multisig contract. If the majority/all agree on the outcome they sign and pay out. All the clever logic is done off chain. It can be anything at all as is not limited by the on-chain script language.

Works today on BTC. Works for any future scripts. All that matters is that those who run the simple multisig contract on-chain are in agreement as to the outcome of the more complex off-chain contract.

If there is an error in the off-chain script, it can be fixed!.. since it is all off chain anyway. They just have to agree.

Quite like it.