'Updates a group.'

def update_group(self, context, group, add_volumes=None, remove_volumes=None):

    for volume in (add_volumes + remove_volumes):
        utils.check_type_matched(volume)
    utils.check_rep_status_matched(group)
    self.add_volumes_to_group_replication(group, add_volumes)
    self.remove_volumes_from_group_replication(group, remove_volumes)
    return self.do_update_cg(group.id, add_volumes, remove_volumes)
