> FWIW, I've been trying to do the same and have been similarly > unsuccessful. I attach a version where I make a shadow copy of Vwindow_list called Vwindow_list_2. The two should behave the same just that Vwindow_list_2 is never accessed by other code. So we can put an assertion like eassert (!NILP (Fequal (Vwindow_list, Vwindow_list_2))); anywhere in the code. I put one in window_list and it should trigger the same way as the length check before. Now if anyone can suggest some strategic positions where to put these assertions, I'll be all ears. martin