public Vec minus(Point p) { return new Vec(x-p.x, y-p.y, z-p.z); }
public Point plus(Vec v) { return new Point(x+v.x, y+v.y, z+v.z); }
public Vec minus(Point p) { return new Vec(x-p.x, y-p.y, z-p.z); }
public Point plus(Vec v) { return new Point(x+v.x, y+v.y, z+v.z); }