@article{CharacterisingTractable,
  title = "Characterising Tractable Constraints",
  author = "M.C.Cooper and D.A.Cohen and P.G.Jeavons",
  year = "1994",
  journal = "Artificial Intelligence",
  number = "65(2)",
  pages = "347--361",
  url = "ftp://ftp.cs.rhul.ac.uk/pub/constraints/characterising.ps",
}