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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 gist(self, id_num):
 """Gets the gist using the specified id number. :param int id_num: (required), unique id of the gist :returns: :class:`Gist <github3.gists.Gist>` """

        url = self._build_url('gists', str(id_num))
        json = self._json(self._get(url), 200)
        return Gist(json, self) if json else None