public double angle(Coord o) {
		Coord c = o.add(this.inv());
		if (c.x == 0) {
			if (c.y < 0)
				return (-PI / 2);
			else
				return (PI / 2);
		} else {
			if (c.x < 0) {
				if (c.y < 0)
					return (-PI + Math.atan((double) c.y / (double) c.x));
				else
					return (PI + Math.atan((double) c.y / (double) c.x));
			} else {
				return (Math.atan((double) c.y / (double) c.x));
			}
		}
	}