docs: def set_event_handler(self, event, handler):
          """Add an Async to be run on event."""
          # QUESTION: Should we raise an exception if `event` is not in some
          # known event-type list?
  
          self._prepare_persistence_engine()
  
          callbacks = self._options.get('callbacks', {})
          callbacks[event] = handler
          self._options['callbacks'] = callbacks