@INPROCEEDINGS{PittsAM:nontpt,
 AUTHOR={A.~M.~Pitts},
 TITLE={Non-trivial Power Types Can't Be Subtypes of 
 Polymorphic Types},
 BOOKTITLE={4th Annual Symposium on Logic in Computer Science},
 PUBLISHER={IEEE Computer Society Press, Washington},
 YEAR=1989,
 PAGES={6--13}}