(define (list-length-iter-cps lst res k0)
  (cond ((null? lst) (k0 res))
        ((pair? lst) (list-length-iter-cps (cdr lst) (+ res 1) k0))
        (else (k0 'improper-list))))

; k0 is passed along the tail recursive calls, and
; can also be used for passing 'an exceptional value'.