(define sum
  (lambda (lst)
    (if (null? lst)
        0
        (+ (car lst)
           (sum (cdr lst))))))

(define length
  (lambda (lst)
    (if (null? lst)
        0
        (+ 1 (length (cdr lst))))))

(define in?
  (lambda (elt lst)
    (cond
      ((null? lst) #f)
      ((equal? (car lst) elt) #t)
      (else (in? elt (cdr lst))))))

(define position-of-in
  (lambda (elt lst)
    (define iter
      (lambda (lst acc)
        (if (in? elt lst)
            (if (equal? elt (car lst))
                acc
                (iter (cdr lst) (+ 1 acc)))
            (error "BK error in position-in, elt not in list" elt lst))))
    (iter lst 0)))