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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 authenticate(self, me, state=None, next_url=None):
 """Authenticate a user via IndieAuth. Args: me (string):
 the authing user's URL. if it does not begin with https?://, http:// will be prepended. state (string, optional):
 passed through the whole auth process, useful if you want to maintain some state, e.g. the starting page to return to when auth is complete. next_url (string, optional):
 deprecated and replaced by the more general "state". still here for backward compatibility. Returns: a redirect to the user's specified authorization url, or https://indieauth.com/auth if none is provided. """

        redirect_url = flask.url_for(
            self.flask_endpoint_for_function(self._authenticated_handler),
            _external=True)
        return self._start_indieauth(me, redirect_url, state or next_url, None)