Exercise 2.57: Extend the differentiation
program to handle sums and products of arbitrary numbers of (two or more)
terms.  Then the last example above could be expressed as


(deriv '(* x y (+ x 3)) 'x)

Try to do this by changing only the representation for sums and products,
without changing the deriv procedure at all.  For example, the
addend of a sum would be the first term, and the augend would be
the sum of the rest of the terms.