code: def add_callback(self, callback):
          
          if callback not in self._callbacks:
              self._callbacks.append(callback)
docs: Add a callback function to the listener.
  
          The callback function will be called for each indication this listener
          receives from any WBEM server.
  
          If the callback function is already known to the listener, it will not
          be added.
  
          Parameters:
  
            callback (:func:`~pywbem.callback_interface`):
              Callable that is being called for each CIM indication that is
              received while the listener threads are running.