@InProceedings{Bac:Wil:WS-CP98,
  author = 	 {Rolf Backofen and Sebastian Will},
  title = 	 {Excluding Symmetries in Concurrent Constraint
                  Programming}, 
  booktitle = 	 {Workshop on Modeling and Computing with Concurrent
                  Constraint Programming}, 
  year =	 1998,
  abstract = {
In many problems, non-determinism naturally arises in an concurrent
constraint-based formulation in form of disjunctive constraints. To
check consistency of disjunctive constraints, one needs to perform
search by splitting the constraint store into local computation
stores.  An important problem there is to eliminate symmetries, which
give rise to many different but ''similar'' solutions found by the
search procedure.  Since symmetries will give rise to an (often
exponential) amplification of the search space, symmetry exclusion
promises to efficiently prune the search tree.

Hence, many constraint programmers have constructed mechanisms to
exclude at least some of the symmetries of their
problem. Unfortunately, symmetry exclusion was often not
straightforward to implement and had to be redesigned for every
special problem or enumeration strategy, which leaded to an
inflexibility in the program structure once it was introduced. Often
symmetry exclusion was not done at all or only for a small set of
symmetries. In other cases, where symmetry exclusion was implemented
it distracted the programmers attention from more important tasks.

The contribution of our work is to give a generally applicable method
for the exclusion of arbitrary symmetries. To our knowledge, this is
the first general and declarative method for excluding symmetries in
concurrent constraint programming.},
  user =	 {will}
}