code: def add_walls(self):
          "Put walls around the entire perimeter of the grid."
          for x in range(self.width):
              self.add_thing(Wall(), (x, 0))
              self.add_thing(Wall(), (x, self.height-1))
          for y in range(self.height):
              self.add_thing(Wall(), (0, y))
              self.add_thing(Wall(), (self.width-1, y)f add_walls(self):
          "Put walls around the entire perimeter of the grid."
          for x in range(self.width):
              self.add_thing(Wall(), (x, 0))
              self.add_thing(Wall(), (x, self.height-1))
          for y in range(self.height):
              self.add_thing(Wall(), (0, y))
              self.add_thing(Wall(), (self.width-1, y))
docs: Put walls around the entire perimeter of the grid.