def comment_thread(cls, backend, *args, **kwargs):
        """Create a comment thread for the desired backend.

        :arg backend:        String name of backend (e.g., 'file',
                             'github', 'redis', etc.).

        :arg *args, **kwargs:   Arguments to be passed to contructor
                                for that backend.

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        :returns:  A CommentThread sub-class for the given backend.

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        PURPOSE:  Some simple syntatic sugar for creating the desired
                  backend.

        """
        ct_cls = cls._known_backends.get(backend)
        if not ct_cls:
            return None
        return ct_cls(*args, **kwargs)