By MIT scheme, do you mean MIT/GNU scheme? A few schemes implement their debuggers in terms of call/cc (or delimited counterparts) and exceptional conditions like errors or breaks hand control to a new 'depth' of REPL where anything in the environment can be inspected (or redefined) in-context before possibly trying to continue or doing any unwinding. If your scheme has no debugging apparatus, it shouldn't be too hard to implement something a little more useful than sprinkling in print statements. Even without wading into continuations, implementing a macro that does tracing should be possible.