形如：
;(let ((variable declare1) (variable declare2) (...))
; (varaible used here));
(defun foo(x)(format t "Parameter: ~a~%" x)(let ((x 2))(format t "Outer LET: ~a~%" x)(let ((x 3))(format t "Inner LET: ~a~%" x))(format t "Outer LET: ~a~%" x))(format t "Parameter: ~a~%" x));(foo 10);
声明的作用域，和C语言很相似，存在覆盖的特点。输出：
Parameter: 10
Outer LET: 2
Inner LET: 3
Outer LET: 2
Parameter: 10
使用let声明时，变量声明域内，无法使用前一个在本声明域内声明的变量：
(defun year-day(y)(let ((m (* y 12)) (d (* m 30)))(format t "Year:~d~%Month:~d~%Day:~d~%" y m d)));
(year-day 1);*** - LET: variable M has no value
使用let*可以进行如此操作：
(defun year-day(y)(let* ((m (* y 12)) (d (* m 30)))(format t "Year:~d~%Month:~d~%Day:~d~%" y m d)));
(year-day 1);
Year:1
Month:12
Day:360
just like this.