Exercise 5. A guest pass for one missing from the registry

An exercise for the closed world page. On the page the accreditation registry is closed: for a known reader, a missing accreditation record becomes a negative fact.

Setup. Introduce a norm “a reader known to the archive but missing from the researcher registry needs a guest pass” and reach three answers on the pass question:

Facts	Answer
known reader, missing from the registry	TRUE_ONLY
known reader, listed in the registry	NEITHER
a person unknown to anyone	NEITHER

Hint. The first row cannot be reached without closure: nothing can establish not in_researcher_registry. What is the closure’s domain, and why is the third row not TRUE_ONLY?

Below is the solution. Try it yourself first.

Solution

Arxo Law

language "law.core" version "0.2";
package tutorial.archive version "0.4.1";
namespace "urn:law:tutorial:archive";

entity Person;

relation known_reader(p: Person) kind institutional;
relation in_researcher_registry(p: Person) kind institutional;
relation guest_pass_required(p: Person) kind institutional;

The researcher-registry closure. The domain is known readers: only about them may the archive say “missing from the registry, hence not listed”:

Arxo Law

closure ResearcherRegistryClosure {
    predicate in_researcher_registry;
    domain known_reader;
    snapshot ARCHIVE_REGISTRY_2026;
    complete_as_of @2026-03-01T00:00:00+00:00;
    derive_explicit_negative true;
}

rule GuestPassRequired strict {
    for p: Person;
    when known_reader(p) and not in_researcher_registry(p);
    then guest_pass_required(p);
}

The setup’s third row is the closure’s boundary. A person outside the domain gets no negative fact: the archive does not claim a random visitor is not a researcher; it simply knows nothing about her. So the norm stays silent instead of requiring a pass from everyone in a row.

Check

The setup’s three rows plus a counterfactual: without the closure declaration the first row becomes NEITHER: with the closure cut out the question is asked again.

What the exercise teaches. A closure is declared on a predicate and bounded by a domain, two separate decisions. The first is which registry is declared complete; the second is exactly about whom. A domainless closure would turn every unknown person into “missing from the registry”, a claim the rules do not contain.