def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def main(self):
      UI.OpenDialog(
        VBox(
          Heading("This Is a Heading."),
          Label("This is a Label."),
          PushButton("&OK")
        )
      )
      UI.UserInput()
      UI.CloseDialog()