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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 sign(self, consumer_secret, access_token_secret, method, url,
             oauth_params, req_kwargs):
        '''Sign request using PLAINTEXT method.

        :param consumer_secret: Consumer secret.
        :type consumer_secret: str
        :param access_token_secret: Access token secret (optional).
        :type access_token_secret: str
        :param method: Unused
        :type method: str
        :param url: Unused
        :type url: str
        :param oauth_params: Unused
        :type oauth_params: dict
        :param req_kwargs: Unused
        :type req_kwargs: dict
        '''
        key = self._escape(consumer_secret) + b'&'
        if access_token_secret:
            key += self._escape(access_token_secret)
        return key.decode()