Exercise 2.87.  Install =zero? for polynomials in the generic arithmetic 
package. This will allow adjoin-term to work for polynomials with 
coefficients that are themselves polynomials.

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(define (poly-equal-zero? p)
  (define (go L)
    (cond ((empty-termlist? L) true)
          ((equ? (coeff (first-term L)) 0) (go (rest-terms L)))
          (else false)))
  (go (term-list p)))