17.4 The natural places where domain-specific planning knowledge can be introduced into our planner are the predicates select and achieves. They select the next goal to be attempted (determining the order in which goals are achieved) and the action to be tried. Redefine these two predicates for the blocks world, so that the goals and actions are more intelligently selected. For this purpose, it is useful to add the current state State as an extra argument to the predicate achieves.