#! implicit Group
struct Group { gid: Option<Int> }
struct User { groups: List<Ref<Group>> = [] }
struct Root {
users: Map<Str, User> = {} block user
groups: Map<Str, Group> = {} block group
}
group wheel { gid = 10 }
user alice { groups = [wheel, alice, "bob"] }
user bob { groups = [bob] }