(define (list-length l)
  (call-with-current-continuation
   (lambda (do-exit)
     (letrec ((list-length1
                (lambda (l)
                   (cond ((null? l) 0)
                         ((pair? l) (+ 1 (list-length1 (cdr l))))
                         (else (do-exit 'improper-list))))))
       (list-length1 l)))  ))