

![0_image_0.png](0_image_0.png)

with the rest of the actions chosen to fatten the pivot branch. Furthermore, we can also see that we have preferred actions using the plane airp1**, since they overlap more with the**
pivot action ap.

Offsetting the Greediness of Fattening: **The fattening procedure is greedy, since it**
insists that the state resulting after fattening have a strictly better heuristic value. While useful in avoiding the addition of irrelevant actions to the **plan, this procedure can also**
sometimes preclude actions that are ultimately relevant but were discarded because the heuristic is not perfect. These actions may then become part **of the plan at later stages** during search (i.e., earlier parts of the execution of the eventual solution plan; since we are searching in the space of plan suffixes). When this happens, the length of the parallel plan is likely to be greater, since more steps that may be needed to **support the preconditions** of such actions would be forced to come at even later stages of **search (earlier parts of the** plan). Had the action been allowed into the partial plan earlier in the search (i.e., closer to the end of the eventual solution plan), its preconditions could probably have been achieved in parallel to the other subgoals in the plan, thus improving **the number of steps.**
In order to offset this negative effect of greediness, AltAltp**re-arranges the partial plan**
to promote such actions higher up the search branch (i.e., later parts of the execution of the eventual solution plan). Specifically, before expanding a given node S, AltAltp**checks**
to see if any of the actions in As leading to S **from its parent node (i.e., Figure 6 shows**
that Apar leads to Spar**) can be pushed up to higher levels in the search branch. This online**