; expt.
(lambda (op one inv) (letrec ((b (lambda (a n)
  (let b ((n n)) (if (< n 4) (if (< n 2) (if (zero? n) one a)
       (if (= n 2) (op a a) (op (op a a) a)))
    (let* ((p (b (quotient n 2)))(p2 (op p p)))
    (if (odd? n) (op a p2) p2)))))))
 (if inv (lambda (a n) (if (< n 0) (inv (b a (- n))) (b a n))) b)))

; tail recursive alternative:
; This code often does one or two unnecessary multiplications.
 (lambda (op one inv) (let ((b (lambda (a n)
  (let E ((a one)(p a)(n n))
  (if (= n 0) a
    (E (if (odd? n) (op a p) a) (op p p) (quotient n 2)))))))
  (if inv (lambda (a n) (if (< n 0) (inv (b a (- n))) (b a n))) b)))

; tests
(let ((pow ((fileVal "expt") * 1 /)))
  (((fileVal "Do") 'DoL) 12 (lambda (j) (pow 2 (- j 2)))))

(let ((pow ((fileVal "expt") * 1 #f)))
  (((fileVal "Do") 'DoL) 12 (lambda (j) (pow 2 j))))

(define p ((fileVal "expt") * 1 /))
(define (lp n) (if (zero? n) '() (cons (p 2 n) (lp (- n 1)))))
(lp 21)