anchor.diff:                                                 (* (y size) 3/4))
                                         (/ (size anim) 2))
                              :image anim
  -                           :health 4 :ammo 400 :missiles 2)))
  +                           :health 16 :ammo 4 :missiles 2)))
               (subscribe (game-keys screen) thopter)
               (incf (players-left (players-left screen)))))
       (let* ((tile-names '(:forest-0 :forest-1 :forest-2 :forest-3
  
positive.code:   (with-slots (level) wave
      (incf level)
      (if (zerop (mod level 10))
          (spawn-wave level (truncate level 10) (* level 20) (* level 4 100)
                      (truncate level 2) (min 1d0 (* level 0.024)) 'enemy-boss)
          (spawn-wave level (+ 2 level) 4 (* (max 1 (ceiling level 3)) 100) 0
                      (min 1d0 (* level 0.11)) 'enemy-ship))))
  
  (defmethod (setf players-left) :after (value (left players-left-controller))
  
negative.code:    (ammo
      :accessor ammo
      :initarg :ammo
      :initform 1)
     (ammo-refill-rate
      :accessor ammo-refill-rate
      :initarg :ammo-refill-rate
      :initform 200)
     (ammo-deplete-rate
      :accessor ammo-deplete-rate
      :initarg :ammo-deplete-rate
      :initform 200)))
  
  (defclass thopter (sprite mobile collidable shooter alarm direction-mixin)
    ((speed :initform 4)
  