I'm not sure how to make the multithreaded case more explicit. The point is that with ownership the multi-threaded case does look exactly like the single thread

I'm not sure how to make the multithreaded case more explicit. The point is that with ownership the multi-threaded case does look exactly like the single threaded case, just with some machinery to "aquire" all the things you want to own snd do the transaction over. But "aquiring" them just looks like having them all in local scope, so there is no explicit `lock`. In the transactional case you mentioned there can still be multiple transactions having access to the same values, so you need the explicit transaction semantics. Ownership never has such cases, unless you use locks to manage ownership between threads, but it could also be done via channels, or CSP, or whatever.