One interesting thing one can do with SETF is define expanders that dive into accessors on "pure" datatypes, copying as needed. Suppose we have a pure cons typ

One interesting thing one can do with SETF is define expanders that dive into accessors on "pure" datatypes, copying as needed. Suppose we have a pure cons type, call it PCONS, with accessors PCAR and PCDR. Then, the form (setf (pcar (pcdr x)) y) turns into (effectively; the actual expansion has temporary variables to avoid duplicating forms) (progn (setq x (pcons (pcar x) (pcons y (pcdr (pcdr x))))) y)