*18.8.2-4

久しぶりになってしまいました。
p250くらいまでやっとこさ辿りつきました。
あー、滅茶苦茶楽しくなってきました（理解度とは比例せず）。。

(define (list=? a-list another-list) ;;17.8.2
  (cond
    ((and (cons? a-list) (empty? another-list)) false)
    ((and (empty? a-list) (cons? another-list)) false)
    (else
     (or (and (empty? a-list) (empty? another-list))
         (or (= (first a-list) (first another-list))
             (list=? (rest a-list) (rest  another-list)))))))

(define sym-list=?  ;;17.1.3
  (lambda (a-list another-list)
    (cond
      ((empty? a-list) (empty? another-list))
      ((cons? a-list)
       (and (cons? another-list)
            (and (symbol=? (first a-list) (first another-list))
        (sym-list=? (rest a-list) (rest another-list))))))))

(define contains-same-numbers  ;;17.8.4
  (lambda (a-lon another-lon)
    (cond
      ((empty? a-lon) true)
      ((and (empty? a-lon) (cons? another-lon)) false)
      ((and (cons? a-lon) (empty? another-lon)) false)
      ((find-n (first a-lon) another-lon)
       (contains-same-numbers (rest a-lon) another-lon)))))

(define find-n 
  (lambda (n lon)
    (cond
      ((empty? lon) false)
      ((= n (first lon)) true)
      (else 
       (find-n n (rest lon))))))