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## 4. The Wave Front

When a layer is reached in which all of the top level goals are pairwise non-mutex Graphplanbased planners begin searching for a plan. If no plan can be found, new layers are constructed alternately with search until the fix point of the graph is reached. In Graphplan and Ipp the graph continues to be explicitly constructed beyond the fix point, even though the layers which can be built beyond this point are sterile (contain no new facts, actions or mutex relations). Their construction is necessary to allow the conditions for achievement of goal sets to be established, between the fix point and the current layer. However, this constitutes significant computational effort in copying existing structures and in unnecessary ss STAN maintains a single layer, called the buffer, beyond the fix point together with a queue of goal sets remaining to be considered. Each time a goal set is removed from this queue, to be considered in the buffer, those goal sets it generates in the fix point layer, which have