@InProceedings{wehr-odersky:proofO,
  author = 	 "Martin Wehr and Martin Odersky",
  title = 	 "Proof of the Principal Type Property for System O",
  booktitle =	 "Technical Report 1996-16 of University of Karlsruhe",
  year =	 1996,
  month =	 "May"
}