code: static function create(string $name, array $settings, string $sql_path) {
          if (isset(self::$_instances[$name]))
              Console::warning("Overwriting migration \"$name\".");
          self::$_instances[$name] = new self($settings, $sql_path);
      }
docs: 新建一个Migrate实例
  
  @param string $name
  @param array $settings
  @param string $sql_path