(define (number-interval f t)
 (if (<= f t)
     (cons f (number-interval (+ f 1) t))
     '()))


(define (number-interval-iter f t)
  (reverse (number-interval-iter-help f t '())))


(define (number-interval-iter-help f t res)
  (if (<= f t)
      (number-interval-iter-help (+ f 1) t (cons f res))
      res))