(define count-down
  (lambda (n)
    (if (zero? n)
        (list 0)
        (cons n (count-down (- n 1))))))

(define count-from
  (lambda (start finish)
    (if (= start finish)
        (list start)
        (cons start (count-from (+ start 1) finish)))))

(define iota-a
  (lambda (n)
    (reverse (cdr (count-down n)))))

(define iota-b
  (lambda (n)
    (count-from 0 (- n 1))))

(define iota-c
  (lambda (n)
    (iota-c-helper 0 n)))

(define iota-c-helper
  (lambda (i n)
    (if (= i n)
        null
        (cons i (iota-c-helper (+ i 1) n)))))

(define iota-d
  (lambda (n)
    (iota-d-helper (- n 1) null)))

(define iota-d-helper
  (lambda (i so-far)
    (if (zero? i)
        (cons 0 so-far)
        (iota-d-helper (- i 1) (cons i so-far)))))