Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  n : nat
  x : A
  Hnth : nth_error (a :: l) (S n) = Some x
  IHl : ∀ n : nat, nth_error l n = Some x → forallb p l → p x
  H : p a
      ∧ (fix forallb (l : list A) : bool :=
           match l with
           | [] => true
           | a :: l0 => p a && forallb l0
           end) l
  ============================
   (is_true (forallb p l))


DEBUG:Going to execute:
exact H || destruct_hyp H
DEBUG LTAC Evaluated term: H
DEBUG LTAC Evaluated term: (p a
                            ∧ (fix forallb (l : list A) : bool :=
                                 match l with
                                 | [] => true
                                 | a :: l0 => p a && forallb l0
                                 end) l)
Level 2: type of H
 has value 
(p a
 ∧ (fix forallb (l : list A) : bool :=
      match l with
      | [] => true
      | a :: l0 => p a && forallb l0
      end) l)