public function add_prefix($field) {
        if (substr($field, 0, 2) === '!_') {
            return '-_' . $this->extraprefix . substr($field, 2);
        } else if (substr($field, 0, 1) === '-') {
            return '-' . $this->extraprefix . substr($field, 1);
        } else if (substr($field, 0, 1) === '_') {
            return '_' . $this->extraprefix . substr($field, 1);
        } else {
            return $this->extraprefix . $field;
        }
    }