This paper describes the DrScheme stepper. The major attraction is the detailed presentation of the (semantics preserving) source transformations (based on continuation marks).
Alas, I don't think the code is available. I'd love to have a look..
A better link would be the Northeastern PLT Publications Page, which I am not able to access at the moment.
Posted to theory by Ehud Lamm on 1/31/02; 11:45:00 AM