code: def unicast(self, socket_id, event, data):
          
          payload = self._server.serialize_event(event, data)
          rv = self._server.sockets.get(socket_id)
          if rv is not None:
              rv.socket.send(payload)
              return True
          return False
docs: Sends an event to a single socket.  Returns `True` if that
          worked or `False` if not.