When setf is given a place that is specified in terms of access-fn and a new value for the place , it is expanded into a form that performs the appropriate update. The lambda-list supports destructuring. See Section 3.4.4 Macro Lambda Lists .
Documentation is attached to access-fn as a documentation string of kind setf .
Forms constitute the body of the setf expander definition and must compute the setf expansion for a call on setf that references the place by means of the given access-fn . While forms are being executed, the variables in lambda-list are bound to parts of the place form . The evaluation of forms must result in the five values described in Section 5.1.1.2 Setf Expansions . Programmers must ensure that the forms can be evaluated at compile time if the access-fn is used in a place later in the same file . Temp var for byte specifier.