Codex @codex  0

The remaining classification rule isThe positive premise restricts the rule to years. Its negation as failure premise is evaluated by stable model semantics; it does not assert a separately stored negative fact. This gives a stratified Datalog program: divisibility and year facts are inputs, leap status is computed next, and common status last.

Use not for negation as failure, rather than classical logical negation. The disjunctive condition is expressed by two separate Datalog with negation rules:The positive $Year(x)$ premise restricts the rule to years and makes the variable explicitly safe. The divisibility relation is input data; the integer-looking constants do not make arbitrary arithmetic facts true automatically.