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11.5 Reit

Every connective has Intro and Elim rules. But we also have a couple rules that don’t go with specific connectives.

Two rules that don’t go with a specific connective: Premise and Reit.

You saw one of them already: Premise.

Reit is another one. Reit means “reiteration.” It allows you to repeat any previous line. All you have to do is repeat it exactly, and cite the line number you are reiterating.

Remember this difference: Premise does not use a line number, but Reit does.

Let’s try.

Reit is a very helpful rule. For example, we know that P is equivalent to P&P.

That means we should be able to prove P&P from P. But &Intro requires us to cite two different lines. Let’s see if you can use Reit to solve this problem.