  between an action a, an initial situation s in which a is performed, and a resulting situation s′ immediately subsequent to the performance of the action. Usually (though this is not absolutely necessary) the deterministic assumption is made that s′ is unique. In general, actions can be successfully performed only under certain limited circumstances. This could be modeled by allowing for cases in which there is no s′ such that RESULT(a,s,s′). But usually, it is assumed that RESULT is in fact a total function, but that in cases in which s does not meet the “preconditions” of a, there are no restrictions on the s′ satisfying RESULT(a,s,s′), so that the causal effects of a will be entirely unconstrained in such cases. 