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### 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.