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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 from_url(cls, reactor, url, key, alg=RS256, jws_client=None):
 """ Construct a client from an ACME directory at a given URL. :param url: The ``twisted.python.url.URL`` to fetch the directory from. See `txacme.urls` for constants for various well-known public directories. :param reactor: The Twisted reactor to use. :param ~josepy.jwk.JWK key: The client key to use. :param alg: The signing algorithm to use. Needs to be compatible with the type of key used. :param JWSClient jws_client: The underlying client to use, or ``None`` to construct one. :return: The constructed client. :rtype: Deferred[`Client`] """

        action = LOG_ACME_CONSUME_DIRECTORY(
            url=url, key_type=key.typ, alg=alg.name)
        with action.context():
            check_directory_url_type(url)
            jws_client = _default_client(jws_client, reactor, key, alg)
            return (
                DeferredContext(jws_client.get(url.asText()))
                .addCallback(json_content)
                .addCallback(messages.Directory.from_json)
                .addCallback(
                    tap(lambda d: action.add_success_fields(directory=d)))
                .addCallback(cls, reactor, key, jws_client)
                .addActionFinish())