Elisp 逻辑运算

and 、 or 、 not 。

or 经常用于设置函数的缺省值，而 and 常用于参数检查：

(defun hello-world (&optional name)
  (or name (setq name "Emacser"))
  (message "Hello, %s" name))           ; => hello-world
(hello-world)                           ; => "Hello, Emacser"
(hello-world "Ye")                      ; => "Hello, Ye"

(defun square-number-p (n)
  (and (>= n 0)
       (= (/ n (sqrt n)) (sqrt n))))
(square-number-p -1)                    ; => nil
(square-number-p 25)                    ; => t