Exercise 4. Suspension also closes the general room

An exercise for the defeat and priority page. On the page a suspended ticket removes rare-collections room access, while the general reading room admits a reader with a suspended ticket because its norm is strict.

Setup. Change the package so that a ticket suspension also removes general-room access. The answer to “is she admitted to the general room” on the facts “registry, ticket suspended” must become NEITHER, and stay TRUE_ONLY without the suspension fact. Separately explain why the answer is not FALSE_ONLY.

Hint. Only a defeasible norm can be defeated. What will the compiler do about it if you forget?

Below is the solution. Try it yourself first.

Solution

Arxo Law

language "law.core" version "0.2";
package tutorial.archive version "0.3.1";
namespace "urn:law:tutorial:archive";

entity Person;

relation in_researcher_registry(p: Person) kind institutional;
relation accredited(p: Person) kind institutional;
relation reader_suspended(p: Person) kind institutional;
relation may_enter_rare_room(p: Person) kind institutional;
relation may_enter_reading_room(p: Person) kind institutional;

rule RareRoomAccess defeasible {
    for p: Person;
    when in_researcher_registry(p) and accredited(p);
    then may_enter_rare_room(p);
}

rule SuspensionQuarantine defeater {
    for p: Person;
    when reader_suspended(p);
    defeat may_enter_rare_room(p);
}

The general-room norm was strict. The first change is making it defeasible, the second adding the defeating norm:

Arxo Law

rule GeneralReadingRoom defeasible {
    for p: Person;
    when in_researcher_registry(p);
    then may_enter_reading_room(p);
}

rule SuspensionClosesReadingRoom defeater {
    for p: Person;
    when reader_suspended(p);
    defeat may_enter_reading_room(p);
}

Why not FALSE_ONLY. A defeating norm removes support rather than asserting the opposite: the archive rules say “the ticket is suspended”, not “entry is banned”. A direct ban, if needed, would be a norm with a not may_enter_reading_room(p) head and priority over the general one, like the debt norm on the page.

Check

Facts	Question	Answer
registry, ticket suspended	general room	NEITHER
registry	general room	TRUE_ONLY
registry, suspended, without the SuspensionClosesReadingRoom norm	general room	TRUE_ONLY

The page is also broken the other way: returning the word strict to the general-room norm must bring the LDC-E4112 refusal from the compiler. A strict head cannot be defeated, and that is caught before any execution.

What the exercise teaches. A norm’s strength is not a matter of taste: a strict norm means “no exceptions”, and any exception first requires changing the strength — a change visible in the diff, requiring grounds in the text.