Exercise 8.14 The selection algorithm used in the partial-order planner is not very sophisticated. It may be sensible to order the selected subgoals. For example, in the robot world, the robot should try to achieve a carrying subgoal before an at subgoal because it may be sensible for the robot to try to carry an object as soon as it knows that it should carry it. However, the robot does not necessarily want to move to a particular place unless it is carrying everything it is required to carry. Implement a selection algorithm that incorporates such a heuristic. Does this selection heuristic actually work better than selecting, say, the last added subgoal?

Can you think of a general selection algorithm that does not require each pair of subgoals to be ordered by the knowledge engineer?