I don't think I understand that example. If I understand correctly, the issue is the question of exclusive mutability, which is why you can't pass the mutable r

I don't think I understand that example. If I understand correctly, the issue is the question of exclusive mutability, which is why you can't pass the mutable reference itself around. But for some reason you can pass around a mutable reference to the collection and an index? That seems surprising: theoretically, if you have a mutable reference to a collection, you can always transform it into a mutable reference to some subset of its children. Or is this a lifetimes issue, where the lifetime of the individual reference might be longer than the lifetime of the collection as a whole? In that case, it might be that the lifetime annotations aren't correct, or that the code is wrong in the first place. I get the idea that you can't change public APIs and function signatures, but that's true in pretty much every language.