anchor.diff:                    (make-instance 'tile
                                    :parent root
                                    :offset (complex x y)
  -                                 :veloc #c(0 1/2)
  +                                 :veloc #c(0 1)
                                    :image (nth (mt19937:random num-tiles)
                                                tiles)
                                    :depth 100))))))
  
positive.code:     (setf (game-wave screen) (make-instance 'wave-controller :parent root)
            (game-quit screen) (make-instance 'quit-controller :parent root)
            (game-background screen)
            (make-instance 'background-controller :parent root :timer 2)
            (players-left screen)
            (make-instance 'players-left-controller :parent root)
            (game-sound screen)
  
negative.code:            (num-tiles (length tiles))
             (tile-size (size (first tiles))))
        (iter (for x from 0 below (x size) by (x tile-size))
              (let ((y (1+ (- (y tile-size)))))
                (make-instance 'tile
                               :parent parent
                               :offset (complex x y)
                               :veloc #c(0 1)
                               :image (nth (mt19937:random num-tiles)
                                           tiles)
                               :depth 100)))
        (setf timer (1- (y tile-size))))))
  
  (defmethod (setf players-left) :after (value (left players-left-controller))
    (when (<= value 0)
  