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:
trivial with eq_true