(define (length3 lst)
  (let ((n 0))
    (while (pair? lst)
      (set! n (1+ n))
      (set! lst (cdr lst)) )
    n))


(define (length4 lst)
  (do ((n 0 (1+ n)))
      ((not (pair? lst)) n)
    (set! lst (cdr lst)) ) )

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