@Article{birkedal+:rel-interp-ic,
  author = 	 {Lars Birkedal and Robert Harper},
  title = 	 {Relational Interpretations of Recursive Types in an Operational Setting},
  journal = 	 {Information and Computation},
  year = 	 {1999},
  volume =	 {155},
  pages =	 {3--63}
}