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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 fork(self):
 """Fork this gist. :returns: :class:`Gist <Gist>` if successful, ``None`` otherwise """

        url = self._build_url('forks', base_url=self._api)
        json = self._json(self._post(url), 201)
        return Gist(json, self) if json else None