In pregel programming model, how does the static method Combine work

[dongxudongxu]

I check your document "06_writing_your_own_algorithms.ipynb", it says

### 可选的 Combiner 函数
我们可以定义一个 Combiner 以减少消息通信的开销，注意这个 Combiner 函数是可选的，您也可以不实现。

It's unclear for me, how to correctly define a Combiner in detail, and how to use it in the static method Compute?
Thanks for your attention and help, looking forward to your reply

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