public function rotate(Point $center, Angle $angle)
	{
		$x = $this->x - $center->x;
		$y = $this->y - $center->y;
		$this->x = $x * $angle->cos - $y * $angle->sin + $center->x;
		$this->y = $y * $angle->cos + $x * $angle->sin + $center->y;
		return $this;
	}