Scieneer Common Lisp 1.3.9 online documentation

step form[Macro]

A debugging paradigm wherein the programmer is allowed to step through the evaluation of a form. We use the debugger's stepping facility to step through an anonymous function containing only form.

Currently the stepping facility only supports stepping compiled code, so step will try to compile the resultant anonymous function. If this fails, e.g. because it closes over a non-null lexical environment, an error is signalled.