Goal:
  
  A : Type
  f : A → bool
  g : nat → A
  n : nat
  fg : ∀ x : nat, x < S n → f (g x)
  IHn : (∀ x : nat, x < n → f (g x)) → forallb f (unfold n g)
  ============================
   (∀ x : nat, x < n → f (g x))


DEBUG:Going to execute:
done
DEBUG LTAC Evaluated term: forallb_app
DEBUG LTAC Evaluated term: forallb_app
DEBUG LTAC Evaluated term: IHn
DEBUG LTAC Evaluated term: IHn
DEBUG LTAC Evaluated term: Prop
DEBUG LTAC Evaluated term: Prop