Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  IHl : ∀ n : nat, nth_error l n = Some a → forallb p l → p a
  H : is_true (p a)
  ============================
   (forallb p l → p a)


DEBUG:Going to execute:
do_intro
Level 4: Backtrace:

Proof is not complete.
Level 4: Backtrace:

No applicable tactic.
Level 3: evaluation returns
idtac of type tacvalue