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:
destruct_hyp H
Level 2: Backtrace:

In environment
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
The term "H" has type
 "p a
  ∧ (fix forallb (l : list A) : bool :=
       match l with
       | [] => true
       | a :: l0 => p a && forallb l0
       end) l" while it is expected to have type "is_true (forallb p l)".
Level 2: Backtrace:

In environment
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
The term "H" has type
 "p a
  ∧ (fix forallb (l : list A) : bool :=
       match l with
       | [] => true
       | a :: l0 => p a && forallb l0
       end) l" while it is expected to have type "is_true (forallb p l)".