SICP 問題 2.91

(define (div-terms L1 L2)
  (if (empty-termlist? L1)
      (list (the-empty-termlist) (the-empty-termlist))
      (let ((t1 (first-term L1))
            (t2 (first-term L2)))
        (if (> (order t2) (order t1))
            (list (the-empty-termlist) L1)
            (let ((new-c (div (coeff t1) (coeff t2)))
                  (new-o (- (order t1) (order t2))))
              (let ((rest-of-result
                     (div-terms (sub-terms L1
                                           (mul-terms L2
                                                      (make-term new-o
                                                                 new-c)))
                                L2)))
                (list (add-terms (list (make-term new-o new-c))
                                 (car rest-of-result))
                      (cadr rest-of-result))))))))

(define (div-poly p1 p2)
  (if (same-variable? (variable p1) (variable p2))
      (make-poly (variable p1)
                 (div (term-list p1)
                      (term-list p2)))
      (error "Polys not in same var -- DIV POLY"
             (list p1 p2))))