Study: Symmetry breaking for ASP
  In their nature configuration problems are combinatorial (optimization)
problems. In order to find a configuration a solver has to instantiate a number
of components of a some type and each of these components can be used in a
relation defined for a type. Therefore, many solutions of a configuration
problem have symmetric ones which can be obtained by replacing some component
of a solution by another one of the same type. These symmetric solutions
decrease performance of optimization algorithms because of two reasons: a) they
satisfy all requirements and cannot be pruned out from the search space; and b)
existence of symmetric optimal solutions does not allow to prove the optimum in
a feasible time.
