anchor.diff:  
   ;;;;2.87
   
  +;;添加到polynomial的算数包中。
  +
  +;;;;2.88
  
positive.code: 					  (add (coeff t1) (coeff t2)))
  			       (add-terms (rest-terms l1) (rest-terms l2)))))))))
  
  (defun mul-terms (l1 l2)
    (if (empty-termlist? l1)
        (the-empty-termlist)
  
negative.code: 			    (mul-terms (term-list p1)
  				       (term-list p2)))
  		 (error "poly not in same variable -- mul-poly ~A" (list p1 p2))))
  	   (sub-poly (p1 p2)
  	     (if (same-variable? (variable p1) (variable p2))
  		 (make-poly (variable p1)
  			    (sub-terms (term-list p1)
  				       (term-list p2)))
  		 (error "poly not in same variable -- mul-poly ~A" (list p1 p2))))
  	   
  	   (tag (p) (attach-tag 'polynomial p)))
      (my-put 'add '(polynomial polynomial)
  	    #'(lambda (p1 p2) (tag (add-poly p1 p2))))
  
      (my-put 'sub '(polynomial polynomial)
  	    #'(lambda (p1 p2) (tag (sub-poly p1 p2))))
      
      (my-put 'mul '(polynomial polynomial)
  	    #'(lambda (p1 p2) (tag (mul-poly p1 p2))))
      ;;2.87 add =zero?
  