public static void reduceToSlag(Box2D area, World world) {
    final int maxSize = (int) Math.max(1, area.xdim() / 2) ;
    
    for (Tile t : world.tilesIn(area, true)) {
      if (t.owner() != null) continue ;
      int size = 2 + Rand.index(maxSize) ;
      
      while (size-- > 1) {
        final Wreckage heap = new Wreckage(false, size) ;
        heap.setPosition(t.x, t.y, world) ;
        if (! heap.area().containedBy(area)) continue ;
        if (! heap.canPlace()) continue ;
        heap.enterWorldAt(t.x, t.y, world) ;
        break ;
      }
    }
  }