<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def friendships_destroy(self, user_id=None, screen_name=None):
 """ Allows the authenticating user to unfollow the specified user. https://dev.twitter.com/docs/api/1.1/post/friendships/destroy :param str user_id: The screen name of the user for whom to unfollow. Required if ``screen_name`` isn't given. :param str screen_name: The ID of the user for whom to unfollow. Required if ``user_id`` isn't given. :returns: A dict containing the newly unfollowed user. """

        params = {}
        set_str_param(params, 'user_id', user_id)
        set_str_param(params, 'screen_name', screen_name)
        return self._post_api('friendships/destroy.json', params)