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