public function add_field($name, $type, $precision=null, $unsigned=null, $notnull=null, $sequence=null, $default=null, $previous=null) {
        $field = new xmldb_field($name, $type, $precision, $unsigned, $notnull, $sequence, $default);
        $this->addField($field, $previous);

        return $field;
    }