(define (f ns)
  (define (aux m1 m2 ns)
    (let* ((n (and (not (null? ns)) (car ns)))
           (ns (and n (cdr ns))))
      (cond
       ((not n) m2)
       ((not m1) (aux n #f ns))
       ((or (and (not m2) (< m1 n)) (< m1 n m2)) (aux m1 n ns))
       ((or (and (not m2) (< n m1)) (< n m1 m2)) (aux n m1 ns))
       (else (aux m1 m2 ns)))))
  (aux #f #f ns))
(print (f '(4 5 1 7 1 2 8 9 2 7)))
(print (f '(1 1 1)))
(print (f '(1)))
(print (f '()))
 

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