new ((A → C) → A) → ((C ∨ A) ∧ C → (C ∧ D → ((A → B) → (C ∧ C) ∧ (D ∨ (D → C)) ∨ A ∧ (D → C))))
intro
intro
cases 1
intro
cases 4
intro
cases 1
cases 4
left
constructor
constructor
exact 3
exact 3
left
exact 6