code: def add_fs(self, name, fs, write=False, priority=0):
          # type: (Text, FS, bool, int) -> None
          
          if isinstance(fs, text_type):
              fs = open_fs(fs)
  
          if not isinstance(fs, FS):
              raise TypeError("fs argument should be an FS object or FS URL")
  
          self._filesystems[name] = _PrioritizedFS(
              priority=(priority, self._sort_index), fs=fs
          )
          self._sort_index += 1
          self._resort()
  
          if write:
              self.write_fs = fs
              self._write_fs_name = name
docs: Add a filesystem to the MultiFS.
  
          Arguments:
              name (str): A unique name to refer to the filesystem being
                  added.
              fs (FS or str): The filesystem (instance or URL) to add.
              write (bool): If this value is True, then the ``fs`` will
                  be used as the writeable FS (defaults to False).
              priority (int): An integer that denotes the priority of the
                  filesystem being added. Filesystems will be searched in
                  descending priority order and then by the reverse order
                  they were added. So by default, the most recently added
                  filesystem will be looked at first.