Swap values of variables and Swap car and cdr of cons cell in Emacs Lisp

swap-value-of-variables.el

(let ((a 1) (b 2))
  (setq a (prog1 b (setq b a)))
  (list a b))
;; => (2 1)

(let ((con '(1 . 2)))
  (setcar con (prog1 (cdr con)
                (setcdr con (car con))))
  con)
;; => (2 . 1)