Re: "chunk of code should be executed atomically" - I'm not 100% sure but I think it's possible that (from the way 'atom is implemented) a sequence that is not protected within an 'atom scope will be able to see an "atomic" operation in progress.
Basically 'atom just acquires a Global Interpreter Lock around the function it executes. I'm not sure, but from the semantics of locking, I think that another thread may see the "atomic" operation step-by-step unless it is itself protected by 'atom (and then there is potential contention for a single global lock!)
Of course, mzscheme uses green threads, so maybe it won't work that way.
