Exercise 2.94: Using div-terms, implement
the procedure remainder-terms and use this to define gcd-terms as
above.  Now write a procedure gcd-poly that computes the polynomial
GCD of two polys.  (The procedure should signal an error if the two
polys are not in the same variable.)  Install in the system a generic operation
greatest-common-divisor that reduces to gcd-poly for polynomials
and to ordinary gcd for ordinary numbers.  As a test, try


(define p1 
  (make-polynomial 
   'x '((4 1) (3 -1) (2 -2) (1 2))))

(define p2 
  (make-polynomial 
   'x '((3 1) (1 -1))))

(greatest-common-divisor p1 p2)

and check your result by hand.