# File karel/tk_robot_world.rb, line 155
  def place_wall_south_of(street, avenue)
        # "Place an east-west wall segment south of this corner"
        legal_corner(street, avenue)
        @east_west_walls[[street - 1, avenue]] = 1
        $window.place_wall_north_of(street - 1, avenue) if $window
  end