example

type Circle struct {
	Center Point
	Radius float64
}

type Point struct {
	X, Y float64
}

type Rectangle struct {
	LowerLeft     Point
	Width, Height float64
}

type Shape interface {

	// Circle returns a Circle and a boolean. When the second return value is true,
	// the Shape is the returned Circle.
	Circle() (Circle, bool)

	// Rectangle returns a Rectangle and a boolean. When the second return value is true,
	// the Shape is the returned Rectangle.
	Rectangle() (Rectangle, bool)
}