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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 ref(self, ref):
 """Get a reference pointed to by ``ref``. The most common will be branches and tags. For a branch, you must specify 'heads/branchname' and for a tag, 'tags/tagname'. Essentially, the system should return any reference you provide it in the namespace, including notes and stashes (provided they exist on the server). :param str ref: (required) :returns: :class:`Reference <github3.git.Reference>` """

        json = None
        if ref:
            url = self._build_url('git', 'refs', ref, base_url=self._api)
            json = self._json(self._get(url), 200)
        return Reference(json, self) if json else None