elisp-misc.el

(setf (buffer-string) (buffer-name))

(setf (mark) 10
      (point) 20)

(setq my-list '(0 1 2 3 4 5))

(cl-letf ((my-list))
  (message "%s" my-list)) ;; ==> "(0 1 2 3 4 5)"

(let ((my-list))
  (message "%s" my-list)) ;; ==> "nil"

(cl-letf (((car my-list) 1000)
          ((elt my-list 3) 200))
  (message "%s" my-list)) ;; ==> "(1000 1 2 200 4 5)"


(cl-letf (((symbol-function 'message) #'ignore))
  (message "hi")) ;; ==> nil

;; hello