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 : is_true (p a)
  H0 : is_true
         ((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:
try (solve [ inversion H ])
DEBUG LTAC Evaluated term: H0
DEBUG LTAC Evaluated term: (is_true
                              ((fix forallb (l : list A) : bool :=
                                  match l with
                                  | [] => true
                                  | a :: l0 => p a && forallb l0
                                  end) l))
Level 4: type of H
 has value 
(is_true
   ((fix forallb (l : list A) : bool :=
       match l with
       | [] => true
       | a :: l0 => p a && forallb l0
       end) l))