Protip: instead of writing

    (a.resource in b.resource and a.action in b.action and a.principal in b.principal) or

You can write

    {
     a.resource in b.resource
     a.action in b.action
     a.principle in b.principle
    } or // ...

(Also instead of `(some principal) iff not (some notPrincipal)` you can write `some principle <=> no notPrinciple`. Alloy has a lot of cool syntactic sugar!)