@article{ClosreProperties,
  title = "Closure Properties of Constraints",
  author = "P.G.Jeavons and D.A.Cohen and M.Gyssens",
  year = "1997",
  journal = "Journal of the ACM",
  number = "44",
  pages = "527--548",
  url = "ftp://ftp.cs.rhul.ac.uk/pub/constraints/closure.ps",
}