@techreport{cjz07:tr-hierarchies,
  title = "The expressive power of valued constraints: {H}ierarchies and collapses",
  author = "Cohen, D.A. and Jeavons, P.G. and \v{Z}ivn\'y, S.",
  year = "2007",
  address = "Oxford, UK",
  institution = "Computing Laboratory, University of Oxford",
  month = "April",
  number = "CS-RR-07-03",
  url = "http://www.cs.ox.ac.uk/files/668/RR-07-03.pdf",
}