class Renderer:
    def render_circle(self, radius):
        raise NotImplementedError
    def render_square(self, side):
        raise NotImplementedError

class VectorRenderer(Renderer):
    def render_circle(self, radius):
        return "vector circle r=" + str(radius)
    def render_square(self, side):
        """Return a vector description of a square of the given side."""
        # TODO: return a string like 'vector square s=4'
        pass

class RasterRenderer(Renderer):
    def render_circle(self, radius):
        return "raster pixels for circle r=" + str(radius)
    def render_square(self, side):
        """Return a raster description of a square of the given side."""
        # TODO: return a string like 'raster pixels for square s=4'
        pass

class Shape:
    def __init__(self, renderer):
        self.renderer = renderer
    def draw(self):
        raise NotImplementedError

class Circle(Shape):
    def __init__(self, renderer, radius):
        super().__init__(renderer)
        self.radius = radius
    def draw(self):
        return self.renderer.render_circle(self.radius)

class Square(Shape):
    def __init__(self, renderer, side):
        super().__init__(renderer)
        self.side = side
    def draw(self):
        """Draw this square by delegating to the bound renderer."""
        # TODO: call self.renderer.render_square(self.side) and return it
        pass