Simplify the solution to prefix matching

[zhouwfang]

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[mgeisler]

Hi @zhouwfang, thanks a lot! This looks much better indeed :smile:

I would be happy to merge this, we just need the CLA to be signed.

[zhouwfang]

Hi @mgeisler, It should be all set now. Thanks.

[mgeisler]

Thanks! Could I also get you to add a few comments to explain the solution? In particular the trick with turning the elements into Option so we can detect when the prefix iterator ends.