部分习题：
exercise 2.73
a)对求导过程来说，数字和变量的表示形式对任何“类型标志”（例如+）都是一致的，没有必要实现不同的表示方式。
b)
1 (define (install-deriv-sum-package) 2 ;internal procedure 3 (define (addend s) (cadr s)) 4 (define (augend s) (caddr s)) 5 (define (make-sum a1 a2) 6 (cond ((=number? a1 0) a2) 7 ((=number? a2 0) a1) 8 ((and (number? a1) (number a2)) (+ a1 a2)) 9 (else (list '+ a1 a2)))) 10 ;interface 11 (put 'addend '+ addend) 12 (put 'augend '+ augend) 13 (put 'make-sum '+ make-sum) 14 (put 'deriv '+ (lambda (exp var) 15 (make-sum (deriv (addend exp) var) 16 (deriv (augend exp) var)))) 17 'done) 18 19 (define (install-deriv-product-package) 20 ;internal pricedure 21 (define (multiplier p) (cadr p)) 22 (define (multiplicand p) (caddr p)) 23 (define (make-product p1 p2) 24 (cond ((or (=number? p1 0) (=number? p2 0)) 0) 25 ((=number? p1 1) p2) 26 ((=number? p2 1) p1) 27 ((and (number? p1) (number? p2)) (* m1 m2)) 28 (else (list '* p1 p2)))) 29 ;interface 30 (put 'multiplier '* multiplier) 31 (put 'multiplicand '* multiplicand) 32 (put 'make-product '* make-product) 33 (put 'deriv '* (lambda (exp var) 34 (make-sum (make-product (multiplier exp) 35 (deriv (multiplicand exp) var)) 36 (make-product (deriv (multiplier exp) var) 37 (multiplicand exp))))) 38 'done)
exercise 2.74
a)人事文件应当有一个tag信息，用于识别该文件属于哪一个分支机构。
exercise 2.75
1 (define (make-from-mag-ang x y) 2 (define (dispatch x y) 3 (cond ((eq? op 'real-part) (* x (cos y))) 4 ((eq? op 'imag-part) (* x (sin y))) 5 ((eq? op 'magnitude) x) 6 ((eq? op 'angle) y) 7 (else (error "Unkown op -- MAKE-FROM-MAG-ANG" op)))) 8 dispatch)
