; arithmetic sequence
 
(define (seq? xs)
  (let* ((mn (apply min xs))
         (mx (apply max xs))
         (len (length xs))
         (diff (/ (- mx mn) (- len 1))))
    (if (not (integer? diff)) #f
      (= (* len (+ len 1) 1/2)
         (apply +
           (map (lambda (x)
                  (if (zero? (remainder x diff))
                      (quotient x diff)
                      0))
                (map (lambda (x) (- x mn (- diff)))
                     xs)))))))
 
(display (seq? '(1 3 5 7 9))) (newline)
(display (seq? '(1 4 5 6 9))) (newline)

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