Highly symmetric problems result in redundant search effort which can render apparently simple problems intractable. Whilst the potential benefits of symmetry-breaking have been explored in the broader search and constraint satisfaction community (see (Gent & Smith 2000; Ip & Dill 1996; Crawford et al. 1996) for examples) there has been relatively little interest in the exploitation of this potential in planning. An initial exploration of the benefits of symmetry-breaking in a Graphplan framework, by Fox and Long in 1999 (Fox & Long 1999) yielded promising results but failed to take into account the importance of identifying and exploiting new symmetries that arise during the search process. In this paper we extend the symmetry exploitation ideas described in (Fox & Long 1999) to handle new symmetries and report results obtained from a range of planning problems.
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