@Article{harper+:girard-j,
  author = 	 {Robert Harper and John C. Mitchell},
  title = 	 {Parametricity and variants of Girard's J operator},
  journal = 	 {Information Processing Letters},
  year = 	 1999,
  volume =	 {??},
  number =	 {??},
  month =	 {??},
  pages =	 {??--??},
  note =	 {(To appear)}
}