def revrank v
      ret = redis.zrevrank rediskey, to_redis(v)
      ret.nil? ? nil : ret.to_i
    end
def revrank v
      ret = redis.zrevrank rediskey, to_redis(v)
      ret.nil? ? nil : ret.to_i
    end
rank of member +v+ when ordered highest to lowest (starts at 0)