(define (negate p)
  (lambda (x) 
    (if (p x) #f #t)))

;; Calculate the factorial of n.
;; .parameter n An integer
;; .pre-condition The integer must be non-negative.
;; .returns n!
(define (fac n)
  (if (= 0 n) 1 (* n (fac (- n 1)))))