@TechReport{HanusPrehofer96,
  author =    "Michael Hanus and Christian Prehofer",
  title =     "Higher-{O}rder {N}arrowing with {D}efinitional {T}rees",
  institution = "RWTH Aachen",
  year =      "1996",
  number =    "AIB-02-1996",
  url =       "http://aib.informatik.rwth-aachen.de/1996/1996-02.ps.gz",
}