Common code from ch2.scm for Exercises 2.87-...
Exercise 2.87: simply implementing (=zero? for polynomial allows for coefficients that are themselves polynomials. But first, I cleaned up Exercise 2.78 to allow for scheme-number coeffs to display much more cleanly
Exercise 2.88 - polynomial subtraction, cheaply
Exercise 2.89 - tiny changes to allow for dense polynomials, but I did peek at the solutions