Having thought about it more, I think it would be fine for `getindex` to throw, since this can happen for uninitialized values in ordinary arrays as well. I think that `show` would also throw, though, which might be problematic. Also `getindex` would throw a `BoundsError()` on an uninitialized array, which isn't particularly intuitive, and to throw a better error message we'd need to make `getindex` do more work. So I guess it's still best to initialize with zeros.