(defun magic-square (a b c d e f g h i)
  (and (/= a b c d e f g h i)
       (= (+ a b c)
          (+ d e f)
          (+ g h i)
          (+ a d g)
          (+ b e h)
          (+ c f i)
          (+ a e i)
          (+ c e g))))