code: func (s *Sprite) In(x, y int) bool {
  	// Check the actual color (alpha) value at the specified position
  	// so that the result of In becomes natural to users.
  	//
  	// Note that this is not a good manner to use At for logic
  	// since color from At might include some errors on some machines.
  	// As this is not so important logic, it's ok to use it so far.
  	return s.image.At(x-s.x, y-s.y).(color.RGBA).A > 0
  }
docs: // In returns true if (x, y) is in the sprite, and false otherwise.