code: def set_stream(self,stream):
          
          _unused = stream
          if self.joined and self.handler:
              self.handler.user_left(self.me,None)
          self.joined=False
docs: Called when current stream changes.
  
          Mark the room not joined and inform `self.handler` that it was left.
  
          :Parameters:
              - `stream`: the new stream.
          :Types:
              - `stream`: `pyxmpp.stream.Stream`