(define nums '(1 2 3 4 5 6 7 8 9))

(define valid
    (lambda (x)
      (foldr
       (lambda (y z)
         (match y
            (nil z)
            (_ (match (filter (lambda (q) (= q y)) nums)
                (((? r) . nil) (match (filter (lambda (q) (eq r q)) x)
                                (((? _)) z)
                                (_ nil)))
                (_ nil)))))
       t
       x)))