Greg Restall on Nostr: (The proof with alternatives does have a bit of a round-about feel, with having to ...

(The proof with alternatives does have a bit of a round-about feel, with having to store the conclusion as an alternative twice to then retrieve both in one go. This gives you the effect of contraction in conclusion position, which is required because the natural deduction introduction rule for disjunction is additive, while the elimination rule is multiplicative. In this format, your only device for contraction in conclusion position is to hoist the conclusion into the assumption context (as an alternative, under the slash) and to then retrieve two or more copies of that assumption back as a conclusion.)

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