anchor.diff:              (incf (players-left (players-left screen)))))
       (let* ((tile-names '(:forest-0 :forest-1 :forest-2 :forest-3
                            :forest-4 :forest-5 :forest-6))
  -           (tiles (iter (for name in tile-names)
  -                        (collect (make-image name))))
  +           (tiles (iter (for name in tile-names) (collect (make-image name))))
              (num-tiles (length tiles))
              (tile-size (size (first tiles))))
         (iter (for x from 0 below (x size) by (x tile-size))
               (iter (for y from 0 below (y size) by (y tile-size))
  -                  (make-instance 'tile
  +                  (make-instance 'sprite
                                    :parent root
                                    :offset (complex x y)
  -                                 :veloc #c(0 1/2)
                                    :image (nth (mt19937:random num-tiles)
                                                tiles)
                                    :depth 100))))))
  
positive.code:       (upgrade-bullet (incf ammo ammo-refill-rate))
        (upgrade-missile (incf missiles))
        (upgrade-speed (progn
                         (setf (speed-boost thopter) 240)
                         (change-veloc thopter event))))
      (when (and parent (<= health 0))
        (make-instance 'explosion :parent parent
  
negative.code:       (setf offset (complex (- (x (size parent)) (x size)) (y offset))))
      (when (> (y offset) (- (y (size parent)) (y size)))
        (setf offset (complex (x offset) (- (y (size parent)) (y size)))))
      (when (and (> (speed-boost thopter) 0) (boost-on thopter))
        (decf (speed-boost thopter))
        (change-veloc thopter event))))
  
  