code: def add_callback(self, fd, events, callback):
          
          if self._mpoll is None:
              raise RuntimeError('Poller instance is closed')
          try:
              mpoll = self._mpoll[fd]
          except KeyError:
              mpoll = self._mpoll[fd] = MultiPoll(self._loop, fd)
          handle = mpoll.add_callback(events, callback)
          return handle
docs: Add a new callback.
  
          The file descriptor *fd* will be watched for the events specified by
          the *events* parameter, which should be a bitwise OR of the constants
          ``READABLE`` and ``WRITABLE``. Whenever one or more of the specified
          events occur, *callback* will be called with a single integer argument
          containing the bitwise OR of the current events.
  
          The return value of this method is an opaque handle that can be used
          to remove the callback or to update the events associated with it.