Acknowledgements

Matthias Felleisen has provided constant support and encouragement for this work. Mark Krentel re-implemented the scanner and reader, enhancing their functionality and improving their performance. He also helped clarify the type hierarchy. Cormac Flanagan and Matthew Flatt are long-suffering and immensely patient users. Matthew has also done outstanding implementation work. Robby Findler has provided valuable support, while Sebastian Good has pushed for improvements. The rest of the PLT Group was a useful (if captive!) audience. Bruce Duba has provided advice and feedback.