public function asLineMarc($sep = '$', $blank = ' ')
    {
        if ($this->field->isEmpty()) {
            return null;
        }
        $subfields = [];
        foreach ($this->field->getSubfields() as $sf) {
            $subfields[] = $sep . $sf->getCode() . ' ' . $sf->getData();
        }
        $tag = $this->field->getTag();
        $ind1 = $this->field->getIndicator(1);
        $ind2 = $this->field->getIndicator(2);
        if ($ind1 == ' ') {
            $ind1 = $blank;
        }
        if ($ind2 == ' ') {
            $ind2 = $blank;
        }

        return "${tag} ${ind1}${ind2} " . implode(' ', $subfields);
    }