To set a breakpoint from the Stepper, position the cursor where you want the breakpoint and choose
Debug > Stepper > Breakpoint
or click 
in the Stepper toolbar.
Figure 26.5 A breakpoint on the function call
+
When you run code, or choose Debug > Stepper > Continue , execution stops if a breakpoint is reached. The Stepper will show the form in the source area with the breakpoint highlighted in yellow.
In the picture above, execution has stopped at the start of the print form and we have just set a breakpoint on the call to
+
. Continuing from this point will cause execution to stop just before it calls
+
, and the Stepper will display the arguments that are about to be passed to
+
.
If you set a breakpoint on the closing parenthesis of a form, then it will cause execution to stop when the form returns and the top backtrace frame will display the values of that form.
To set a breakpoint from the Editor, see Breakpoints.
LispWorks IDE User Guide (Windows version) - 25 Nov 2011