anchor.diff:    ;at present, damage is dealt when the shockwave has reached half its maximum radius.
     (when (and (> (* 2 (radius explosion))
                   (damage-radius explosion))
  -             (not (damage-dealt-p explosion))) 
  +             (not (damage-dealt-p explosion)))
         (deal-damage explosion))
     (when (> (radius explosion) (damage-radius explosion))
         (invalidate  explosion)))
  -       
  +
   (defmethod tick ((explosion explosion))
     (setf (radius explosion)
             (+ (radius explosion) *speed-of-sound*)
  
positive.code:                          :destination (rallying-point factory))
            (current-cost factory)
            (cost (first (queue factory)))))
                           
    (when (>= (progress factory)
              (current-cost factory))
      (setf (current-cost factory)
  
negative.code:              1.2) ;selection indicator must be larger than the unit
            (rot (selected entity))
            (+ 2 (rot (selected entity)))))
  
    (grid-update entity)
    (call-next-method))
  
  