Mersenne: LL question

[Spike Jones]

OK I understand how you start with 4, then square and
subtract 2, then take the mod M, then repeat P times.
If the remainder is 0, then M is prime.

But what if the mod M comes out to 1 on one of
the intermediate steps?  Then 1^2 - 2 = -1
Then what?  spike

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