public function cotan(int $scale = null): Decimal
    {
	    $sin = $this->sin($scale + 2);
	    if ($sin->isZero()) {
	        throw new \DomainException(
	            "The cotangent of this 'angle' is undefined."
	        );
	    }

	    return $this->cos($scale + 2)->div($sin)->round($scale);
    }