Goal regression ($\mathrm{Regress}(G,a)=(G\setminus \mathrm{Add}(a))\cup \mathrm{Pre}(a)$)

For a positive conjunctive goal $G$, an action with $\mathrm{Del}(a)\cap G=\varnothing$ 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.