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In this paper we explore the relative importance of persistent and non-persistent mutex relations in the performance of Graphplan- based planners. We also show the advantages of pre-compiling persistent mutex relations. Using TIM we are able to generate, during a pre-processing analysis, all of the persistent binary mutex relations that would be inferred by Graphplan during each graph construction. We show how the efficient storgae of, and access to, these pre-processed persistent mutexes yields a modest improvement in graph construction performance. We further demonstrate that the process by which these persistent mutexes are identified can, in certain kinds of domain, allow the exploitation of binary mutex relations which are inaccessible to Graphplan. We present The Island of Sodor, a simple planning domain characterizing a class of domains in which certain persistent mutexes are present but are not detectable by Graphplan during graph construction. We show that the exploitation of these hidden binary mutexes makes problems in this kind of domain trivially solvable by STAN, where they are intractable for other Graphplan-based planners.
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