code: def comment_thread(cls, backend, *args, **kwargs):
          
          ct_cls = cls._known_backends.get(backend)
          if not ct_cls:
              return None
          return ct_cls(*args, **kwargs)
docs: 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.