says: choose arbitrarily which of the atoms formula_1 to include in the stable model. The lparse program that contains this choice rule and no other rules has 8 stable models—arbitrary subsets of formula_2. The definition of a stable model was generalized to programs with choice rules. Choice rules can be treated also as abbreviations for propositional formulas under the stable model semantics. For instance, the choice rule above can be viewed as shorthand for the conjunction of three "excluded middle" formulas: