sicp-ex-2.94

  
  
 (define (greatest-common-divisor a b) 
   (apply-generic 'greatest-common-divisor a b)) 
  
 ;; add into scheme-number package 
 (put 'greatest-common-divisor '(scheme-number scheme-number) 
      (lambda (a b) (gcd a b))) 
  
 ;; add into polynomial package 
 (define (remainder-terms p1 p2) 
   (cadr (div-terms p1 p2))) 
  
 (define (gcd-terms a b) 
   (if (empty-termlist? b) 
     a 
     (gcd-terms b (remainder-terms a b)))) 
  
 (define (gcd-poly p1 p2) 
   (if (same-varaible? (variable p1) (variable p2)) 
     (make-poly (variable p1) 
                (gcd-terms (term-list p1) 
                           (term-list p2)) 
     (error "not the same variable -- GCD-POLY" (list p1 p2))))) 
  
 (put 'greatest-common-divisor '(polynomial polynomial) 
      (lambda (a b) (tag (gcd-poly a b)))) 

result of (greatest-common-divisor p1 p2): -x^2+x