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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 append_user_agent(self, user_agent):
 """Append text to the User-Agent header for the request. Use this method to update the User-Agent header by appending the given string to the session's User-Agent header separated by a space. :param user_agent: A string to append to the User-Agent header :type user_agent: str """

        old_ua = self.session.headers.get('User-Agent', '')
        ua = old_ua + ' ' + user_agent
        self.session.headers['User-Agent'] = ua.strip()