GNU Emacs Lisp Reference Manual
You can modify the car and cdr contents of a cons cell with the
primitives setcar and setcdr.
Common Lisp note: Common Lisp uses functionsrplacaandrplacdto alter list structure; they change structure the same way assetcarandsetcdr, but the Common Lisp functions return the cons cell whilesetcarandsetcdrreturn the new car or cdr.