Exercise 2.56.  Show how to extend the basic differentiator to handle 
more kinds of expressions. For instance, implement the differentiation 
rule



by adding a new clause to the deriv program and defining appropriate 
procedures exponentiation?, base, exponent, and make-exponentiation. 
(You may use the symbol ** to denote exponentiation.) Build in the rules 
that anything raised to the power 0 is 1 and anything raised to the 
power 1 is the thing itself.

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(define (deriv exp var)
  (cond ...

        ((exponentiation? exp)
         (let ((n (exponent exp))
               (u (base exp)))
           (make-product (make-product n (make-exponentiation u (- n 1)))
                         (deriv u var))))
        ...))

(define (exponentiation? x)
  (and (pair? x) (eq? (car x) '**)))

(define (base exponentiation) (cadr exponentiation))

(define (exponent exponentiation) (caddr exponentiation))

(define (make-exponentiation base exponent)
  (cond ((=number? base 1) 1)
        ((=number? base 0) 0)
        ((=number? exponent 0) 1)
        ((=number? exponent 1) base)
        ((and (number? base) (number? exponent)) (expt base exponent))
        (else (list '** base exponent))))