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Datalog with negative body premises. Its meaning depends on the specified semantics, such as stable model semantics or evaluation of a stratified Datalog program.

A negative rule premise interpreted through a specified rule semantics rather than as an independently asserted classical negative fact. Stable model semantics evaluates such premises through a candidate interpretation and its stable model reduct.

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.

A consequence holds skeptically if it is true in every admitted model, in particular every stable model when using stable model semantics. Exhibiting one supporting stable model establishes only existence of support, not skeptical entailment.