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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 init_app(self, app):
 """Initialize an application. :param app: A :class:`~flask.Flask` app. """

        if not hasattr(app, 'extensions'):
            app.extensions = {}

        app.extensions['nav'] = self
        app.add_template_global(self.elems, 'nav')

        # register some renderers
        for args in self._renderers:
            register_renderer(app, *args)