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It's worth mentioning that Segwit2x can only exist if it has a majority of the hashrate. If your definition of Bitcoin is the post-fork chain with the most hashrate, then Segwit2x will either be Bitcoin or it won't exist. It's only if your definition of Bitcoin is Bitcoin Core that Segwit2x can be an altcoin.
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Yes Segwit2x is an example of knowing the major fork in advance, it won't activate until it is the major fork, so should be no issue for miners on p2pool ... unless they want to back a centrally controlled minor fork ...