27.3 The implementation of the Stepper
It is important to understand the following points about the implementation of the Stepper.
27.3.1 Requirements for stepping
The code you step must have been compiled, evaluated or loaded in the Lisp image.
27.3.2 Editing source code
While the Stepper is running, it displays a read-only copy of the source in the source area. Therefore, you cannot edit the code in the source area, other than when the status is "Enter a form to step in the pane above.".
If you step a function for which the source has been edited since it was compiled, then the Stepper uses a copy of the compile-time source, not the edited source.
This copy is stepped in a new editor buffer created specially for it and this is displayed in the source area.
27.3.3 Side-effects of stepping
When the Stepper steps a definition for the first time, it evaluates it.
This will not normally alter the behavior of your program, but there are three situations where this will cause unexpected behavior:
The code is loaded from a fasl file which is not compatible with the corresponding source file.
The source relies on compile-time side-effects of forms preceding it in the file.
The defining form has other side effects. This is unlikely to matter for simple definers such as
27.3.4 Atomic and constant forms
It is not possible to step to atomic forms or constant forms.
LispWorks IDE User Guide (Unix version) - 12 Feb 2015