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

(define integers-from-to
  (lambda (low high)
    (if (> low high)
        '()
        (cons low  
              (integers-from-to (+ 1 low) high)))))

(define integers-from-to  ; faulty version
  (lambda (low high)
    (define iter
      (lambda (low lst)
        (if (> low high)
            lst
            (iter (+ 1 low)
                  (cons low lst)))))
    (iter low '())))

(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)))