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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def register_app(self, name: str, app: Callable = None) -> Callable: """ register dispatchable wsgi application"""

        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