Also, as I was writing this, a couple things occurred to me:

* We _could_ use the function parameter syntax `entities: &world.entities[]` instead of `entities in world.entities[]`.

* "Groups" aren't really central to understanding the idea, so Path Borrowing might be a better name than Group Borrowing.

Opinions welcome =)

FWIW `foo in bar` implies iteration to me, not a path. In the .NET world, for example, `bar` would be a collection that we’re iterating through, assigning each element to `foo` in turn.