@InCollection{Kent:2002:inOCL,
  author =       "Stuart Kent and John Howse",
  title =        "Constraint Trees",
  booktitle =    "Object Modeling with the OCL: The Rationale behind the
                  Object Constraint Language",
  publisher =    "Springer",
  year =         "2002",
  editor =       "Tony Clark and Jos Warmer",
  pages =        "228--249",
  isbn =         "3-540-43169-1",
}