defsetf

(defsetf access-fn update-fn)

Registers a setf expansion for access-fn. The short form above makes (setf (access-fn args...) value) expand to (update-fn args... value). The long form (defsetf access-fn (lambda-list) (store-var...) body...) evaluates its body at expansion time -- the lambda list bound to temporaries for the argument forms and the store variables bound to the new-value temporaries -- and must return the store form. For the full five-value protocol use define-setf-expander. Works on every backend.