new ((B → C) → A) → (D → (B → (A ∧ (C ∧ A) → (A → D ∨ D) ∨ A) ∨ (D → B)))
intro
intro
intro
left
intro
cases 3
cases 3
cases 5
cases 7
left
intro
cases 3
cases 5
cases 7
cases 14
left
exact 1