anchor.diff:  (define (null? x) (eq? x nil))
   (define (not a) (if a nil T))
   (define (to-float a) (* a 1.0))
  +
  +(define (exp a n)
  +  (if (> n 0)
  +    (exp-pos a n)
  +    (/ 1 (exp-pos a (- 0 n)))))
  +
  +(define (exp-pos a n)
  +  (if (= 0 n)
  +    1
  +    (* a (exp a (- n 1)))))
   
   (defmacro (eval-cond c . cx)
     `(if ,c
  
positive.code:   ;; Defines the greater-than operator
  (define eq =)
  (define (> a b) (if (< a b) nil T))
  (define (abs x) (if (< x 0) (- 0 x) x))
  
negative.code: (define string> string-greaterp)
  
  ;;; list based arithmetic
  (define string-concat
    (let ((old-sc string-concat))
      (lambda xs (foldl old-sc "" xs))))
  
  (define +
    (let ((old+ +))
      (lambda xs (foldl old+ 0 xs))))
  