 where \(A\) is an action variable, and \(\gamma_{f}^+ (\boldsymbol{x}, A)\) and \(\gamma_{f}^- (\boldsymbol{x}, A)\) are ‘simple’ expressions without action modalities expressing the conditions for \(\phi\) becoming true and false, respectively. So the formula is read informally as, under certain preconditions pertaining to the action \(A\) at hand, the fluent (predicate) \(f\) becomes true of arguments \(\boldsymbol{x}\), if and only if either the condition \(\gamma_{f}^+ (\boldsymbol{x}, A)\) holds or \(f(\boldsymbol{x})\) holds (before the execution of \(A)\) and the condition \(\gamma_{f}^- (\boldsymbol{x}, A)\) (that would cause it to become false) does not hold. Furthermore, the expression \(\Poss(A)\) is used schematically in such axioms, where the whole action theory should be complemented with so-called precondition axioms of the form \(\phi_A \rightarrow \Poss(A)\) for concrete expressions \(\phi_A\) stating the actual preconditions needed for a successful execution of \(A\).