The Stepper tool allows you to follow the execution of your program, displaying the source code as it executes. While stepping, you can see the evaluation of each subform, function call and the arguments and return values in each call. At every call to one of your functions, you have the option of stepping into that function, that is stepping the source code definition of the function.
Where a macro appears in stepped code, the Stepper can macroexpand the form and step the resulting expansion, or simply step the visible inner forms of the macro form. Where a special form such as
if
appears in stepped code, the Stepper processes it according to the execution order in that special form.
The system creates a Stepper tool automatically when your code reaches a breakpoint
Other ways to start a Stepper tool are:
LispWorks IDE User Guide (Macintosh version) - 25 Nov 2011
in the LispWorks toolbar and enter a single form