public function removeSubfield (\HAB\Pica\Record\Subfield $subfield) 
    {
        $index = array_search($subfield, $this->_subfields, true);
        if ($index === false) {
            throw new InvalidArgumentException("Cannot remove subfield: Subfield not part of the subfield list");
        }
        unset($this->_subfields[$index]);
    }