Glossary

Plain-language definitions of the terms used in these pages.

Atom. A constant name in Prolog: alice, light_on, 'Hello'. Distinct from numbers: '1' is an atom, 1 is an integer.

Backtracking. When a line of reasoning fails, the engine goes back to the most recent choice it made and tries the next option. This is how Prolog finds every answer.

Built-in. A predicate the engine provides rather than one you define: member/2, findall/3, is/2.

Clause. A fact or a rule.

Cut (!). "Commit to the choices made so far." Once a cut runs, Prolog won't try this predicate's remaining clauses, or other answers for the goals before the cut in this clause. Useful, and notoriously easy to get subtly wrong.

Event Calculus. A way of reasoning about time, introduced by Kowalski and Sergot in 1986. This engine uses a simplified form from the 1990s. Events happen at time points and initiate or terminate fluents. A fluent keeps holding until something terminates it; that persistence is called inertia.

Fact. A statement taken as true: parent(alice, bob).

Fluent. A fact that can change over time: light_on, device_mode(vend_01, offline).

Left recursion. A rule whose first goal calls the rule itself, as in path(X,Y) :- path(X,Z), edge(Z,Y). Plain Prolog loops forever on it; tabling fixes that.

Mutation testing. Deliberately breaking the code in small ways to check that some test notices. A test that never goes red can't be protecting anything.

Negation as failure (\+). \+ G succeeds when G can't be proven. "Not provable" isn't quite "false," which is why it needs care.

Oracle. In testing, the source of truth for expected answers. Here, usually SWI-Prolog.

Predicate. A named relation with a number of arguments, written name/arity: parent/2.

Prolog. A logic programming language from 1972. You describe facts and rules, then ask questions; the system searches for answers.

Provenance. Where a fact came from. Here, the sentence in the source story that supports it.

Result status. In the rewrite, a label on each query result: complete (the search finished), exhausted (it stopped early, because a step or depth budget ran out or a circular question was stopped, so the truth is unknown) or cancelled (the host asked it to stop). A game decision that isn't complete is indeterminate. A complete result with no answers means "false." It can also mean "no such predicate": by default, unknown predicates quietly fail.

Rule. A conditional fact: ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z). reads "X is an ancestor of Z if X is a parent of some Y and Y is an ancestor of Z."

SLD resolution. The standard way Prolog executes: take the first goal, find a matching clause, replace the goal with that clause's body, and repeat, backtracking on failure.

Stratification. A check that no predicate depends on its own negation, directly or indirectly. A program that passes has a single intended model, meaning one agreed set of true facts. This engine refuses programs that don't pass.

SWI-Prolog. A widely used, mature, free Prolog system. It's used here as the referee.

Tabling. Remembering the answers to a subgoal and reusing them. Recursive rules that would otherwise loop forever then finish with every answer, as long as there are finitely many different calls and answers.

Unification. Making two terms equal by binding variables: parent(X, bob) unifies with parent(alice, bob) by setting X = alice.

Variable. A placeholder, written with a capital letter or underscore: X, Who, _.