# File karel/tk_robot_world.rb, line 414
  def save_world(filename)
    file = File.new(filename, "w")
    file.puts "KarelWorld"
    @beepers.keys.each do |key|
       put_line(file, "beepers", key[0], key[1], @Beepers[key])
    end
    @east_west_walls.keys.each{|key| 
      put_line(file, "eastwestwalls", key[0], key[1], key[1])
    }
    @north_south_walls.keys.each{|key|  
      put_line(file, "northsouthwalls", key[1], key[0], key[0])
    }
    file.close
    nil
  end