Given a list of flat Option objects which may or may not have
        'backend' in their kwargs, return a list of grouped backend

def _expand_by_backend(cls, options, backend):
        """
        Given a list of flat Option objects which may or may not have
        'backend' in their kwargs, return a list of grouped backend
        """
        groups = defaultdict(list)
        used_fallback = False
        for obj in options:
            if 'backend' in obj.kwargs:
                opts_backend = obj.kwargs['backend']
            elif backend is None:
                opts_backend = Store.current_backend
                obj.kwargs['backend']= opts_backend
            else:
                opts_backend = backend
                obj.kwargs['backend'] = opts_backend
                used_fallback = True
            groups[opts_backend].append(obj)

        if backend and not used_fallback:
            cls.param.warning("All supplied Options objects already define a backend, "
                              "backend override %r will be ignored." % backend)

        return [(bk, cls._expand_options(o, bk)) for (bk, o) in groups.items()]