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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 create_ref(self, ref, sha):
 """Create a reference in this repository. :param str ref: (required), fully qualified name of the reference, e.g. ``refs/heads/master``. If it doesn't start with ``refs`` and contain at least two slashes, GitHub's API will reject it. :param str sha: (required), SHA1 value to set the reference to :returns: :class:`Reference <github3.git.Reference>` if successful else None """

        json = None
        if ref and ref.count('/') >= 2 and sha:
            data = {'ref': ref, 'sha': sha}
            url = self._build_url('git', 'refs', base_url=self._api)
            json = self._json(self._post(url, data=data), 201)
        return Reference(json, self) if json else None