Yep, that works today. The neat thing here is that `world` is immutable _except_ for its `.entities[]` elements.

That wouldn't be shareable with another thread, because part of it is `mut`. _Theoretically_ we could make this shareable with other threads if we had another kind of effect, let me know if you're curious about that.

The `entity in world.entities[] mut` gives `step` blanket permission to mutate any element inside world.entities. We wouldn't be able to say "all other entities are readonly".

Though, it's worth mentioning that `advance` and `resolve` both receive the entity as a unique reference (since it's the only reference into a group which has a mut effect).