5.1.2.9 Other Compound Forms as Places

For any other compound form for which the operator is a symbol f, the setf form expands into a call to the function named (setf f). The first argument in the newly constructed function form is newvalue and the remaining arguments are the remaining elements of place. This expansion occurs regardless of whether f or (setf f) is defined as a function locally, globally, or not at all. For example,

(setf (f arg1 arg2 ...) new-value)

expands into a form with the same effect and value as

 (let ((#:temp-1 arg1)          ;force correct order of evaluation
       (#:temp-2 arg2)
       ...
       (#:temp-0 new-value))
   (funcall (function (setf f)) #:temp-0 #:temp-1 #:temp-2...))

A function named (setf f) must return its first argument as its only value in order to preserve the semantics of setf.