(setf circ (list 'foo))
(setf (cdr circ) circ)

circ

;;;  setf modifies "pointers"
;;;  and getter returns a reference
;;;  so using setf with getter as "place" works as a setter
;;;
;;; append - nondestructive
;;; nconc - destructive merging of lists

(nconc '(hello lists) '("and" "with" other 15 "values"))

;; subst - nondesctructive
;; nsubst - destructive

(setf tree '(i say (e i (e i) o)))
(subst 'a 'e tree)
tree

(nsubst 'a 'e tree)
(nsubst 'cherry '(a i) tree :test #'equal)
;; this modifies the list

;;; exercise,
;; chop - redule non nil list to list of head
(defun chop (x)
  (setf (cdr x) nil))

(setf my-list '(1 2 "hello"))
(chop my-list)
my-list

(defun ntack (x a)
  (setf (cdr (last x)) (list a)))
(setf my-test-ntack-list '(1 2 "hello"))
(ntack my-test-ntack-list 'boo)
my-test-ntack-list

;; and more recursion
(setf h '(hi ho))
(setf (cdr (last h)) h)

;;; setq - before "generalized variables", only sets value of ordinary variables
(setq x '(slings and arrows))
(setq (cdr x) (list "hello"))           ; variable name is not a symbol