docs: def on(self, event):
          """
          Returns a wrapper for the given event.
  
          Usage:
  
              @dispatch.on("my_event")
              def handle_my_event(foo, bar, baz):
                  ...
  
          """
          handler = self._handlers.get(event, None)
          if not handler:
              raise ValueError("Unknown event '{}'".format(event))
          return handler.register