(define fn/in (lambda (a b k n)
  (if (eqv? k n)
    (begin (display "got it: ") (display a) (newline))
    (if (not (null? b))
      (begin (fn/in (cons (car b) a) (cdr b) (+ 1 k) n)
             (fn/in a (cdr b) k n))))))

(define fn (lambda (l n)
  (fn/in '() l 0 n)))

(fn (list 9 1 3 2 5) 4)