(define (list-length lst)
  (catch 'exit
    (letrec ((list-length1
               (lambda (lst) 
                 (cond ((null? lst) 0)
                       ((pair? lst) (+ 1 (list-length1 (cdr lst))))
                       (else (throw 'exit 'improper-list))))))
       (list-length1 lst))))