code: def register_app(self, name: str, app: Callable = None) -> Callable:
          
          if app is None:
              def dec(app):
                  """ inner decorator for register app """
                  assert app is not None
                  self.register_app(name, app)
                  return app
              return dec
          self.applications[name] = app
          return None
docs: register dispatchable wsgi application