anchor.diff:                (make-instance enemy-type :parent root :depth 1
                                :health health
                                :firepower firepower
  +			     :missiles missiles
  +			     :missile-chance missile-chance
                                :difficulty (floor wave 5))
               (setf offset
                     (complex (+ (/ (x root-size) 2) (* i (x size) 2))
  
positive.code:     (incf level)
      (if (zerop (mod level 10))
          (spawn-wave level (truncate level 10) (* level 20) (* level 4)
                      'enemy-boss)
          (spawn-wave level (+ 2 level) 4 (max 1 (ceiling level 3))
                      'enemy-ship))))
  
  ;; For interactive use:
  (defun thopter ()
  
negative.code: (defmethod collide ((missile missile) event)
    (with-slots (parent offset size depth veloc) missile
      (when (and parent (or (and (typep missile 'missile)
                                 (typep (event-hit event) 'enemy))
                            (typep (event-hit event) 'explosion)))
        (let ((explosion (make-instance 'anim :name :explosion)))
          (make-instance 'explosion :parent parent
  