5.6 Modifying Existing List Structure

You can modify the CAR and CDR contents of a cons cell with the primitives setcar and setcdr. These are destructive operations because they change existing list structure.

Common Lisp note: Common Lisp uses functions rplaca and rplacd to alter list structure; they change structure the same way as setcar and setcdr, but the Common Lisp functions return the cons cell while setcar and setcdr return the new CAR or CDR.