new (A → B) → (G → (A ∨ G → (A ∨ F → (((B → E) → C) → (D → (B ∨ F) ∨ C)))))
intro
intro
intro
intro
intro
intro
left
cases 2
left
apply 0
exact 6
cases 3
left
apply 0
exact 8
right
exact 9