Exercises marked as "challenge problem" are optional: no credit, but you'll deepen your understanding of things.

Additional exercise, on paper

Turn a sheet of paper sideways (landscape mode). Write this on the bottom line:

⊢ i<6 ∨ A → ( ¬ A → i<6)

Above it, write a proof in natural deduction, using these rules.

To make things more convenient, you may use this form of the Axiom rule:

Γ, A, Γ′ ⊢ A

Scan your sheet of paper and turn it in as proof.pdf.