Goal regression
= Goal regression
{title2=$\operatorname{Regress}(G,a)=(G\setminus\operatorname{Add}(a))\cup\operatorname{Pre}(a)$}
For a positive conjunctive goal $G$, an action with $\operatorname{Del}(a)\cap G=\varnothing$ can serve as the final step. Before it executes, the remaining goals and its <preconditions> must hold, giving the displayed regressed set. For <precondition-free STRIPS planning>, only deletion of already supplied requirements remains.