(let ((a (list 2 1)))
  (sort a #'<)
  a)
;; => (2)

(let ((a (list 1 2)))
  (sort a #'<)
  a)
;; => (1 2)

;; ちゃんとsort結果を束縛しような！
(let* ((a (list 2 1))
       (b (sort a #'<)))
  b)
;; => (1 2)