Exploiting symmetry in the planning graph via explanation-guided search

We present a method for exploiting the symmetry in the planning graph structure and certain redundancies inherent in the Graphplan algorithm, so as to improve its backward search. The main insight underlying our method is that due to these features the backward search conducted at level k + 1 of the graph is essentially a replay of the search conducted at the previous level k with certain well-defined extensions. Our method consists of maintaining a pilot explanation structure capturing the failures encountered at previous levels of the search, and using it in an intelligent way to guide the search at the newer levels. The standard EBL and DDB techniques can be employed to control the size of the pilot explanation. The technique has been implemented in the EGBG system, and we present a preliminary empirical study.