docs: def rem_callback(self, event, cb):
          '''Remove a callback from this node.
  
          The callback is removed from the specified event.
  
          @param cb The callback function to remove.
  
          '''
          if event not in self._cbs:
              raise exceptions.NoSuchEventError(self.name, event)
          c = [(x[0], x[1]) for x in self._cbs[event]]
          if not c:
              raise exceptions.NoCBError(self.name, event, cb)
          self._cbs[event].remove(c[0])