It fits in a (new) tweet. Prove these equivalent: em = ∀ {A : Set} → A ⊎ ¬ A dne = ∀ {A : Set} → ¬ ¬ A → A peirce = ∀ {A B : Set} → ((A → B) → A) → A iad = ∀ {A B : Set} → (A → B) → ¬ A ⊎ B dem = ∀ {A B : Set} → ¬ (¬ A × ¬ B) → A ⊎ B
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The answer doesn't fit in a tweet. But it's five proofs, three of which are two lines long, two of which are four lines long. Really lovely.
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Even better: with a couple of imports of work already done in the course, each of the five proofs can be reduced to two lines.
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Which means the answer still won't fit in a new tweet. But it will fit into two!
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Another benefit is now it's clear this is a reasonable exercise in Proust. It wasn't clear looking at my Coq solutions. So into the LACI flânerie it will go.
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