code: def next(self):
          
          loop = self.get_io_loop()
          return self._framework.run_on_executor(loop, self._next)
docs: Advance the cursor.
  
          This method blocks until the next change document is returned or an
          unrecoverable error is raised.
  
          Raises :exc:`StopAsyncIteration` if this change stream is closed.
  
          You can iterate the change stream by calling
          ``await change_stream.next()`` repeatedly, or with an "async for" loop:
  
          .. code-block:: python3
  
            async for change in db.collection.watch():
                print(change)