A user may only rent one copy of the same movie.

context User
inv: rentedCopy->isUnique(movie)

Alternative:¹⁾

context User
inv: rentedCopy.movie->asSet() = rentedCopy.movie

Only 5% of a store's inventory may be in the hands of untrusted or underaged users.

contex Store
inv: owner->select(!user.trusted or user.age < 18)->size() / owner.user->size() <= 0.05