In the stretch command. I'm sorry, I forgot to translate in english language. In my code, the user selects the polyline, inputs the desired area, then selects the vertices to be stretched. The rules for stretching every vertex are defined in the cond statement.
I am a beginner in lisp, so I may be wrong and that's why I need your help. If you can, I would like you to run my code and see the cause of that error.
Cab, is not necessary to preserve adjacent segments lengths, they can be modified.