setf from cl-macs.el from Emacs (elisp source for setf macro)

gistfile1.lisp

(defmacro setf (&rest args)
  "Set each PLACE to the value of its VAL.
This is a generalized version of `setq'; the PLACEs may be symbolic
references such as (car x) or (aref x i), as well as plain symbols.
For example, (setf (cadar x) y) is equivalent to (setcar (cdar x) y).
The return value is the last VAL in the list.

\(fn PLACE VAL PLACE VAL ...)"
  (if (cdr (cdr args))
      (let ((sets nil))
	(while args (push (list 'setf (pop args) (pop args)) sets))
	(cons 'progn (nreverse sets)))
    (if (symbolp (car args))
	(and args (cons 'setq args))
      (let* ((method (cl-setf-do-modify (car args) (nth 1 args)))
	     (store (cl-setf-do-store (nth 1 method) (nth 1 args))))
	(if (car method) (list 'let* (car method) store) store)))))