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

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

An exercise for the [closed world](/tutorials/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

```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":

```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.