簡單的 LOOP 形式

沒有特殊關鍵字的簡單 LOOP 表單：

(loop forms...)

為了擺脫迴圈，我們可以使用 (return <return value>)`

一些例子：

(loop (format t "Hello~%"))  ; prints "Hello" forever
(loop (print (eval (read)))) ; your very own REPL
(loop (let ((r (read)))
        (typecase r
         (number (return (print (* r r))))
         (otherwise (format t "Not a number!~%")))))