anchor.diff:      (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))))
  +		    (truncate level 5) (min 1d0 (* level 0.02)) 'enemy-boss)
  +        (spawn-wave level (+ 2 level) 4 (max 1 (ceiling level 3)) 0
  +                    (min 1d0 (* level 0.01)) 'enemy-ship))))
   
   ;; For interactive use:
   (defun thopter ()
  
positive.code: 
  (defmethod collide ((missile enemy-missile) event)
    (with-slots (parent offset size depth veloc) missile
      (when (and parent 	  (and (typep missile 'enemy-missile)
  			       (typep (event-hit event) 'thopter)))
        (let ((explosion (make-instance 'anim :name :explosion)))
          (make-instance 'explosion :parent parent
                         :offset (+ offset (/ size 2) (/ (size explosion) -2))
  
negative.code:    (missiles
      :accessor missiles
      :initarg :missiles
      :initform 0)
     (missile-chance
      :accessor missile-chance
      :initarg :missile-chance
      :initform 0)
     (difficulty
      :initarg :difficulty
  