public function toNQuads()
    {
        if ($this->isConcrete()) {
            if ($this->isQuad()) {
                return $this->getSubject()->toNQuads().' '.
                       $this->getPredicate()->toNQuads().' '.
                       $this->getObject()->toNQuads().' '.
                       $this->getGraph()->toNQuads().' .';
            } else {
                return $this->getSubject()->toNQuads().' '.
                       $this->getPredicate()->toNQuads().' '.
                       $this->getObject()->toNQuads().' .';
            }
        } else {
            throw new \Exception('A Statement has to be concrete in N-Quads.');
        }
    }