class Vector2d extends AbstractVector implements Serializable {

    public double angleBetween(Vector2d otherVector)
    {
        // This is faster than Math.acos(dot(otherVector)) and can also produce
        // negative angles.
        return Math.atan2(otherVector.myY, otherVector.myX) - Math.atan2(myY, myX);
    }

    public  Vector2d(double x, double y);
    public  Vector2d(Vector2d vec);
    public  Vector2d(Vector2i vec);

}