(cdr a)错，不可对atom。(cdr (a))=nil

cdr, car只处理非空list

cons将原子加到list的前头;将任何S表达式整个加到list前头

(cons a (b))=(a b)
错(cons (a) b)

（null? ())=true用来判断list是否为空（是否含有S表达式），不可用于原子

#lang racket
(define (atom? x)
(and (not (null? x))
(not (pair? x))))
(define lat?
(lambda (l)
(cond
((null? l) #t )
((atom? ( car l)) (lat? (cdr l)))
(else #f ) ) ) )

#lang racket
;whether atom a is a member of list lat
(define member?
(lambda (a lat)
(cond ((null? lat) #f)
(else (or (eq? (car lat) a)
(member? a (cdr lat)))))))

else is a question whose value is always true

过程定义 (define (square x) (* x x))
其中x为参数
(define (sum-of-square x y) (+ (square x) (square y)))

代换模型，将实值一层层地向上代换掉形参，人类的做法，无法被数学严格定义，因为用作过程中形参的名字会与表达式中的名字混淆。但是解释器不是如此。正则序求值：完全展开而后归约；目前LISP用应用序求值，先求值参数而后应用，避免对表达式重复求值。

#lang racket
( define (abs x)
(cond ((> x 0) x)
((< x 0) (- x))
((= x 0) 0))
)
(define (>= x y)
(or (> x y) (= x y))
)

(define (>= x y)
(not (< x y) )
)

输入三个数，输出最大两数之和
#lang racket
(define (sum-of-max x y z)
(if(> x y)
(if(> y z) (+ x y) (+ x z))
(if(> x z) (+ x y) (+ y z))
)
)

牛顿法计算立方根
#lang racket
;cube root
(define (cube-root x)
(cube-iter 1.0 x)
)
(define (cube-iter guess x)
(if (good-enough? guess x)
guess
(cube-iter (improve guess x) x)
)
)
(define (improve guess x)
(get-result guess x)
)
(define (get-result x y)
(/ (+ (/ y (* x x)) (* 2 x)) 3)
)
(define (good-enough? guess x)
(< (abs (- (* guess guess guess) x)) 0.0001)
)