There are ways of making a DVCS that allow all merges to be associative, and all patches commutative except when there's a causal dependency between them, e.g.

There are ways of making a DVCS that allow all merges to be associative, and all patches commutative except when there's a causal dependency between them, e.g. if patch A creates a file, and patch B edits that file, then they cannot commute. I believe darcs makes these guarantees, and making a correct implementation is relatively straightforward. (Making it fast is more complicated, but definitely doable.) Ultimately, though, what you really want is for the VCS to just do what you mean. That's a lot trickier than providing mathematical guarantees about patch reordering and convergence.