... to step through code ... this has to be told at compilation time. The :step option could be reimplemented by forcing the function to be re-compiled, though.

Thanks Giovanni,
of cource I appreciate ECL better performance and indeed I switched from CLISP to ECL for that reason and also because of better Windows support.

Unfortunately I also need the stepper sometimes to debug my code. Recompiling is not a problem at all. But how do I enable the stepping feature on a function when compiling? I mean, is that already implemented? Do I have to implement it by myself in Lisp itself? :-)