# File karel/tk_robot_world.rb, line 148
  def place_wall_west_of(street, avenue)
        # "Place an north-south wall segment east of this corner"
        legal_corner(street, avenue)
        @north_south_walls[[street, avenue - 1]] = 1;
        $window.place_wall_east_of(street, avenue - 1) if $window
  end