层次性结构

(define (append a b)  (if (null? a)      b      (cons (car a) (append (cdr a) (cons (car a) b)))))(define nil (list))(define (count-leaves x)  (cond ((null? x) 0)        ((not (pair? x)) 1)        (else (+ (count-leaves (car x))                 (count-leaves (cdr x))))));;(define (reverse l)  (define (reverse-iter tmp ans)    (if (null? tmp)        ans        (reverse-iter (cdr tmp) (cons (car tmp) ans))))  (reverse-iter l nil));2.27(define (deep-reverse l)  (define (deep-reverse-in tmp ans)    (cond ((null? tmp) ans)          ((pair? (car tmp))           (deep-reverse-in (cdr tmp)                            (cons (deep-reverse-in (car tmp) nil) ans)))          (else (deep-reverse-in (cdr tmp)                                 (cons (car tmp) ans)))))  (deep-reverse-in l nil));2.28(define (fringe t)  (define (fringe-in tmp ans)    (cond ((null? tmp) ans)          ((not (pair? tmp)) (cons tmp ans))          (else (fringe-in (car tmp) (fringe-in (cdr tmp) ans)))))  (fringe-in t nil))(fringe (list (list 1 (list 5 6 7)) (list 3 4) 8));2.29(define (make-mobile left right)  (cons left right))(define (make-branch length structure)  (cons length structure))(define (left-branch x)  (car x))(define (right-branch x)  (cdr x))(define (branch-length x)  (car x))(define (branch-structure x)  (cdr x))(define (total-weight t)  (if (not (pair? t))      t      (+ (total-weight (branch-structure (left-branch t)))         (total-weight (branch-structure (right-branch t))))))(define (mobile-balance t)  (or (not (pair? t))      (and (mobile-balance (branch-structure (left-branch t)))           (mobile-balance (branch-structure (right-branch t)))           (= (* (total-weight (branch-structure (left-branch t)))                 (branch-length (left-branch t)))              (* (total-weight (branch-structure (right-branch t)))                 (branch-length (right-branch t)))))))